Preheat oven to 350 degrees F and grease a 8 x 8 baking dish.
Bring a large pot of water to boil. When the water is boiling, add a healthy pinch of salt and add rigatoni. Stir well and cook according to package instructions. When done, drain (do not rinse) and set aside.
Meanwhile, in a medium saucepan, heat olive oil and saute onion and garlic until fragrant, about 2 minutes.
Add sliced mushrooms and continue to saute until mushrooms are just soft, but still firm, about 7 minutes, stirring occasionally.
Add diced tomatoes and bring to a gentle simmer. Add salt and pepper to taste, simmer for 2 more minutes.
Pour cooked rigatoni into the baking dish and pour sauce over the top. Mix slightly by stirring a few times with a wooden spoon. Top with grated parmigiano cheese.
Place in oven, uncovered, and cook for 15 - 20 minutes until cheese is melted and dish is hot and bubbly. Serve with a side salad and a glass of red wine. Makes a hearty main dish.