Bring a large pot of lightly salted water to a boil. Add rigatoni, and cook until al dente, 8 to 10 minutes; drain.

Meanwhile, melt 1 tablespoon butter in a large skillet over medium heat. Stir in the garlic, onion, and bell pepper. Cook and stir until the onion has softened and turned translucent, about 5 minutes. Add the chorizo slices, and continue cooking until the chorizo is no longer pink in the center, about 10 minutes.

Melt the remaining 2 tablespoons butter in a saucepan over medium heat. Whisk in the flour, and cook for 5 minutes, stirring constantly. Slowly whisk in the milk, and bring to a simmer over medium-high heat. Add the diced tomatoes, salt, and pepper, then reduce the heat to medium-low, and simmer 10 minutes to thicken and smooth the sauce. Stir the cooked chorizo into the sauce along with the drained rigatoni pasta. Stir, and sprinkle with Parmesan cheese to serve.