Preheat the oven to 220 degrees C. Cook the pasta in boiling water for 10–12 minutes, or according to the packet instructions, until almost al dente. Add the peas for the final 2 minutes of cooking. Drain the pasta and peas well.
Heat the oil in a heavy-based saucepan and cook the red capsicum for 1–2 minutes. Add the mushrooms and cook for 2–3 minutes or until softened, stirring occasionally.
Stir in the flour, then gradually stir in the milk and bring to the boil, stirring. Simmer until thickened.
Add the mustard and blue vein cheese with pepper to season, and stir until the cheese has melted. Add the pasta and peas and mix thoroughly. Pour the mixture into an ovenproof dish.
Mix the Cheddar cheese with the breadcrumbs and sprinkle this over the pasta mixture. Bake for 10–15 minutes or until lightly browned and bubbling hot. Serve immediately.