How to Make It
Heat the olive oil in a large skillet over moderately high heat. Add the onion and cook, stirring occasionally, until softened, 6 to 8 minutes. Stir in the red pepper flakes, then add the spinach and cook, stirring, until just wilted, about 3 minutes. Add the ricotta and lemon zest and season with salt and pepper. Keep warm over very low heat.
Meanwhile, cook the spaghetti in a large pot of boiling well-salted water until al dente. Set aside 1 cup of the pasta cooking water, and then drain the spaghetti well. Add the spaghetti to the skillet and toss to coat, adding some of the pasta cooking water to loosen the sauce as needed. Serve immediately.