Add the toasted pine nuts, lemon juice, garlic, sea salt, and a few grinds of black pepper to a food processor, then pulse until the mixture is finely chopped. The texture should look coarse and damp, almost like wet sand, with no big chunks of garlic left behind.

Add the basil leaves and pulse again just until the leaves are broken down and mixed in. The mixture should turn bright green and look evenly chopped rather than fully pureed.

With the machine running, pour in the olive oil in a slow stream, then pulse in the Parmesan cheese, if using, just until combined. The pesto should look creamy and loosely textured, and you can add a bit more olive oil if you want it smoother.