Start cooking the water for the pasta. Speed up the process by using hot water from the tap or boiling from the kettle. When it boils, add salt generously. Cook pasta according to the packet’s instructions, it should be al dente, not mushy. Drain and reserve about ½ cup cooking water (you will not need it all, use as required). (Note)
Squeeze the lemon, grate the garlic and chop the parsley.
When the pasta is almost ready whisk the tahini, lemon juice, garlic, chili, some salt, and pepper. The sauce will seize at first. Slowly add some of the reserved cooking water, about 5-6 tablespoons or enough to thin the sauce a little.
Adjust the taste with more lemon juice, salt, and pepper if necessary. Chop the parsley and add it to the sauce.
Combine the pasta with the sauce, taste, and adjust again if necessary.
Peas (about 1 cup): add them to the pasta pot during the last 3-4 minutes of the cooking time.
Mushrooms (about 250 g/ 8.8 oz) or zucchini (1 medium): slice and fry them in a pan with a little olive oil while the pasta cooks.
Spinach: add a couple of handfuls of baby spinach to the very hot, just drained pasta. The heat will help it wilt.
Roasted veggies of any kind: start preparing them before cooking the pasta, as they will need some time in the oven.