In a heavy-bottomed saucepan, heat olive oil over medium-low heat. Add onion and saute until transparent.
Add diced zucchini, season with salt and pepper, and cover pan. Leave lid on and let zucchini cook over low heat for 15 minutes.
Bring a pot of salted water to boil.
After 15 minutes, remove lid from pan and raise heat to medium-high. Saute, stirring frequently until liquid has evaporated. Do not overcook zucchini.
Cook penne rigate pasta in boiling water until al dente. Drain and add to pot with zucchini.
Toss pasta and zucchini together in pot and add ricotta and Parmigiano-Reggiano. Gently fold ingredients together until cheeses are evenly distributed.
Serve hot. Sprinkle with remaining Parmigiano-Reggiano to taste.