Preheat the oven to 375 degrees F. Cook the pasta in a large pot, according to the package directions. Drain and return to the pot.
While the pasta is cooking, in a small pan, melt 1 teaspoon of butter over medium heat. Add onions and peppers and cook slowly, stirring occasionally, for about 8 minutes, until vegetables are tender but not browned. Remove from heat.
In a medium saucepan, over medium heat, melt 4 tablespoons of butter. Once the butter is melted, quickly whisk in the flour, and cook the roux, stirring constantly, until it turns a pale golden brown.
Add the milk to the saucepan, whisking constantly so that lumps don't form. Once the mixture is smooth, bring to a simmer, whisking frequently. The mixture will begin to thicken as it starts to bubble.
Add the cheese and continue to whisk until melted and smooth. Remove from heat.
Pour the sauce into the pot with the pasta, add the vegetables, and stir until well combined. Butter a 2-quart glass baking dish, and pour the pasta mixture into the dish, spreading it evenly.
Melt 2 tablespoons of butter in a small bowl and toss in the bread crumbs. Mix well and sprinkle the breadcrumbs evenly over the top of the pasta. Sprinkle a little cayenne pepper over the breadcrumbs if you like, and bake uncovered in the preheated oven for around 25 minutes. Let cool for about 5 minutes before serving.