In a large pot of boiling, salted water, cook the pasta until al dente; drain and return to the pot.
Meanwhile, in a large skillet, heat the olive oil over medium heat; add the onion and garlic and cook until the onion is softened, about 5 minutes. Add the spinach by the handful and cook, stirring, until wilted, 3 to 4 minutes.
Add the spinach mixture, ricotta and 3/4 cup parmigiano-reggiano to the pasta and toss. Serve immediately with more parmigiano-reggiano.