Preheat the oven to 350°F (175°C) and prepare a 3-quart baking dish by lightly greasing it with olive oil.

Bring a large pot of salted water to a boil and cook the pasta for two minutes less than the recommended time. Reserve 1/2 cup of the pasta water before draining.

Clean the mushrooms with a damp paper towel, discard any hard stems, and chop into 1/4-inch slices or cubes.

Heat a wide oven-safe skillet over medium-high heat, add the oil, then add the mushrooms and cook until browned for 3 to 5 minutes.

Stir in the onions and peppers, and cook for another 3 to 5 minutes until softened with browned edges. Season with salt and black pepper.

Stir the fennel, pasta sauce, and basil into the vegetables.

If the skillet is large enough, toss in the drained pasta. If not, combine the mixture in a bowl and adjust with reserved pasta water, then add to a baking dish.

Taste the pasta and adjust with more salt and pepper. Tuck spoonfuls of ricotta around the pasta and vegetables, then top with mozzarella cheese.

Bake uncovered until the cheese melts and is bubbling, about 20 to 25 minutes.