In Genoa, Italy, the birthplace of pesto, it goes without saying that the sauce is made with basil. Genoese basil, to be exact. Pesto Genovese is so big there that the airport had to loosen its rules, allowing travelers to bring more than 3-ounces of liquid in their carry-on baggage, providing that liquid is pesto (they screen it with their breast milk and medicine scanner).
Every Genovese family will have its own recipe, but the further you get from Cristoforo Colombo Airport, it's the rules of pesto themselves that loosen. Other types of basil aren't frowned upon with such animation. Here in the New World, it isn't a given that pesto contains basil, and chefs have taken to making a big deal out of that fact by tacking on "pesto" as a suffix to the name of the pureed non-basil leafy green du jour.
And there are many. I did a web search for "pesto recipe –basil”, which screens out any hits that mention "basil", and found recipes for pesto made from a variety of leaves. They included the usual suspects like parsley, cilantro, spinach, kale, garlic scapes and arugula, not to mention edgier foliage options like dill, mizuna, onion tops, beet greens, fennel greens, turnip greens, collard greens, chard, broccoli, watercress, and, in season, asparagus.
In other words, you can essentially toss the whole garden into your food processor, add olive oil, garlic, cheese and nuts. And presto, you've got pesto. You can do the same with many of the so-called weeds many people pull from their gardens, the dandelion, plantain, purslane and lambs quarter, as well as the wild plants growing in your neighborhood, like nettles, wild mustard, ramps and miner's lettuce. And you can even do the same thing with many of the items you would have put in the compost pile, like celery leaves, turnip greens, radish leaves and carrot tops. I even found a recipe for carrot peel pesto. But that, to me, is going too far.
The word "pesto" comes from the Italian pestare, which means "to crush, grind, pound." It's derived from Latin pisto, which means "I pound." In addition to being the root of "pesto," this etymology also gives us the word "pestle," which was, along with the mortar, the original tool of choice for making pesto.
But does that mean that we can really start referring to mashed carrot peels as "pesto?"
Only if I can start calling mayonnaise an egg "pesto."
After all, just like pesto, the special crème was originally made in a mortar and pestle. And lo and behold, mayo is really good on pasta. But if it doesn’t have chlorophyll, I can’t call it pesto.
It was a batch of spinach pesto that solidified my thinking. I made it because I had too much spinach on my hands, and pesto has a way of making large piles of leaves become very small. This batch, made with olive oil, Parmesan and cashews, was oddly satisfying, despite the fact that the flavor of spinach is so much subtler than that of basil. But spinach is about as high in chlorophyll as a leaf can get, and the resulting pesto, a dark, deep shade of green, was full of it. Since then, maxing out the chlorophyll density has been my goal when making pesto.
When I recently followed a recipe for romaine lettuce pesto, I found the result completely unsatisfying. So I added some dark leaves of kale and chard and got it back on track. Another time I made a batch of radicchio pesto. It was purple and creamy and bitterly delicious, but I couldn’t call it pesto. It lacked the mineral embrace of green plant blood.
Basil is a wonderfully aromatic vessel for chlorophyll and is probably still my favorite leaf from which to make pesto, but spinach is a close second. After that, I prefer the weeds, like lambs quarter, or wild plants like nettles, both of which have bold, chlorophyll-dense flavors. But mixing and matching your leaf species adds complexity to the pesto, and is highly recommended.
When basil is in season, I focus on that, and make enough to freeze for year-round use. While I typically add nuts, garlic and cheese to my fresh pesto, when I make it for storage I keep it very simple: just olive oil, basil and salt. I don't skimp on the olive oil, neither in quality nor quantity. The pesto should be fluid enough to set off an airport liquid detector, after all.
So I add enough oil to create a smooth vortex in the food processor, so the basil gets pureed as quickly as possible. I don't skimp on the oil, because I don’t want my pesto to look like a black half-digested hairball the cat spat up. Add enough for a smooth easy vortex, with salt to taste.
I pack my basil and oil puree into half-pint mason jars, and freeze them. When it’s time to make pesto I let them thaw out slowly in the fridge before using.
To convert this freezer-stable basil mixture into proper pesto, I make a paste of the missing ingredients, nuts, cheese and garlic, and mix that with my thawed basil puree. It makes sense to leave these final ingredients out of the freezer until needed, because their flavors are much more vivid when used fresh.
The nuts one can use are as diverse as the foliage options. Pine nuts are the classic, Genoese choice, and you cannot go wrong with them. But walnuts, pistachios, hazelnuts, macadamia nuts, cashews, pecans and almonds are excellent as well. I recently made a delicious batch with cashews and Italian lemon almonds. It took the place of grating a little lemon zest to my pesto. And since lemons are in season during winter, that's another fresh touch you can add to your stored basil in the off season.
Making fresh pesto in summer is almost the same as resurrecting frozen pesto in winter. Start with some garlic (say a clove) and salt in the food processor (or pestle), with enough oil for an easy whizz. Then add nuts and a few scrapes of lemon zest, and whizz some more. Finally, pack in a big handful of leaves, a quarter cup of grated Parmesan or similar cheese, add a half-cup of olive oil, and whizz it all together quickly.
You should play around with any and all of the proportions, not to mention the various combinations of leaves and nuts, and come up with the recipe that suits you and your cooking habits. Just as, in Genoa, every home has a different pesto recipe, so it could be in your own home, with your own local, leafy vessels of chlorophyll.