Get the pasta water going. In the meantime, break off any hardy stems from the kale leaves. Using your hands, rip the kale leaves into big pieces and toss in the food processor. Add the parsley and any other herbs or spinach, and then either add the garlic clove put through a garlic press or minced by hand. Add a bit of salt, the lemon juice, and start the food processor. With the motor running, drizzle in the olive oil slowly. Once the mixture has come together, add the walnuts and lemon zest, if desired, and then process. You can add a Tb. or so of water to get the mixture smooth. Taste the mixture for salt and set aside.

Once the pasta water is going, cook the spaghetti. When the pasta is done, save a few tablespoons of the hot, starchy pasta water. Drain the pasta and then toss it with the pesto, adding the hot pasta water if needed to evenly coat the pasta with the pesto. Toss in the crumbled feta and serve while hot.