Bring a large pot of salted water to a rolling boil. Add fettuccini or linguine, and cook according to package instructions until al dente, about 8–10 minutes. Reserve 1/2 cup of the cooking water, then drain the pasta and set it aside.

In a large skillet, melt 2 tablespoons of butter over medium heat. Add lemon slices and sear them until golden, about 1 minute per side. Remove, chop, and mix with chopped basil for topping.

In the same skillet, add the remaining butter and melt. Add onions, sauté until caramelized and golden, about 8–10 minutes. Stir in garlic, zucchini, thyme, salt, and pepper, cooking until zucchini is tender, about 5 minutes.

Stir in reserved pasta water and heavy cream into the skillet. Simmer over medium heat until slightly thickened, about 5 minutes. Mix in Parmesan cheese and stir until melted.

Add the drained pasta to the skillet, tossing until well coated with sauce. Serve immediately with the lemon-basil mixture on top and extra Parmesan if desired.