Preheat the oven to 350 degrees F. Spray a 9x13 inch baking dish with nonstick spray.
Bring a pot of salted water to a boil and cook the pasta according to the directions. I like to cook it al dente since we are baking the mac and cheese.
Heat the butter in a saucepan over medium heat. Once melted, whisk in the flour to create a roux. Cook for 2 to 3 minutes, until golden brown and fragrant. Slowly pour in half of the milk, whisking constantly so the mixture can thicken, about 5 minutes. Slowly pour in the other half of the milk and whisk until thickened. Stir in the garlic and a pinch of salt and pepper. Reduce the heat to low and add most of the cheese (the mozz, cheddar and parm) - I reserve a few ounces of the mozzarella and cheddar for topping. Stir the cheese sauce until smooth and silky. At this point, you can taste and season the sauce with more salt and pepper if needed.
Place the cooked pasta in the baking dish and stir in the chopped roasted red peppers. (As a note, you can add more toppings INSIDE the mac and cheese - I prefer to keep mine on top.) Pour the cheese sauce all over the pasta and toss it well to combine. Add your toppings on next! The green peppers, olives, pepperoni, banana peppers, etc. Sprinkle with the breadcrumbs.
Bake for 30 to 35 minutes, until golden and bubbly on top. Top with extra grated parmesan and red pepper flakes before serving.