Cook the pasta shells in a saucepan of lightly salted boiling water for 8–10 minutes, or according to the packet instructions, until al dente.
Meanwhile, heat the oil in a large frying pan. Add the onion and fry gently for 3 minutes. Add the peppers and garlic and carry on frying, stirring frequently, for 5 minutes. Stir in the tomatoes and fry for 1 minute until they are soft.
Melt the butter in another pan, toss in the breadcrumbs and stir until all the bread is covered in butter.
Drain the pasta, add the pepper and tomato mix, and then the tuna. Mix together, then put in an ovenproof dish.
Sprinkle the mozzarella or Gruyère and then the buttered breadcrumbs over the pasta and cook under a medium grill for 3–5 minutes until the cheese has melted and the breadcrumbs are golden.