Back when I lived in New York, if I left home for a week, I'd come back to an apartment full of half-dead plants. These days, living in San Mateo, when I leave home for a week (like I did last week), I come home to a garden that's gone full-on feral. My tomatoes overthrew the local government and claimed a bed for themselves. (I summarily executed every remaining lettuce plant after harvesting what the tomatoes left me.) Butternut squash the size of basketballs appeared out of nowhere. Cucumbers went from Starburst-sized nubs to overgrown behemoths threatening to break under their own weight. Massive piles of zucchini spontaneously generated from among the chaos, and I wound up with enough pesto from bolting basil plants to sauce more pasta than I could ever possibly eat.
With a container full of pesto and a pile of zucchini and lettuce at my disposal, I did the only reasonable thing by putting them all together. The result is this creamy, bright summer soup, packed with flavor from a dollop of pesto stirred into it just before serving.
To make it, I start by sautéing onions, carrots, and celery together in butter, along with some coriander seeds and fennel seeds, cooking it all until the vegetables are softened. Next, I add some chopped zucchini, followed by lettuce, a handful at a time, letting each handful of lettuce wilt before adding the next. (Spinach or arugula would be a great substitute.) To give the soup a little more body, I also add a handful of dried long-grain rice. After you add water and a splash of heavy cream to the pot, the rice tenderizes and subsequently thickens the soup when you blend it all together. The rice-thickened-soup trick is one of my favorites, as I've always got dried rice on hand, and it's a really simple way to add body to a puréed soup without having to add too much extra butter or cream.
I like to purée some basil and lemon zest in with the soup as well, to give it a hit of freshness right at the end.
Once it's seasoned to taste, the soup is quite delicious on its own, but stirring in that pesto makes it even better. (I use Daniel's method, which involves pounding the basil, garlic, and pine nuts using a mortar and pestle. It's the best way to make pesto—at least, if good flavor is what you're after.)
Incidentally, stirring pesto into soup is nothing radical. In Provence, pistou, a condiment virtually identical to Ligurian pesto (though it lacks pine nuts and cheese), is most frequently seen stirred into a minestrone-like vegetable soup. When it's not coating my pasta or topping my pizza, pesto finds its way into my soup bowl more often than not.