In a large pot of salted boiling water, cook the mezzi rigatoni according to package instructions until al dente. Reserve about 1 cup of pasta water, then drain the pasta and set it aside.
In a skillet over medium heat, add the olive oil. Once hot, add the chopped onion and sauté until translucent, about 3-4 minutes. Stir in the minced garlic and sliced mushrooms, cooking for an additional 5-6 minutes until the mushrooms are tender and have released their moisture.
Reduce the heat to low and add the Boursin cheese to the skillet. Stir until the cheese has melted and is well combined with the vegetables. If the mixture is too thick, gradually add some reserved pasta water until you reach the desired creaminess.
Add the fresh spinach to the skillet and cook until wilted, about 2 minutes. Season with salt and black pepper, adjusting to taste.
Add the cooked pasta to the skillet, tossing to coat it evenly with the creamy Boursin sauce. If needed, add more reserved pasta water to help combine everything nicely.
For the topping, heat a small skillet over medium heat. Add a drizzle of olive oil and the bread crumbs, toasting them until golden brown and crunchy. Sprinkle them over the pasta before serving.