Preheat the oven to 375°F. Grease an 8x8-inch or 2-quart baking dish.
Cook the pasta in salted boiling water for 1–2 minutes less than the package directions. Drain.
Melt 2 tablespoons of the butter in a large saucepan over medium heat. Whisk in the flour and cook for 1 minute. Slowly whisk in the milk and simmer for 3–4 minutes, until slightly thickened.
Remove from the heat. Stir in the salt, pepper and 1 cup of the cheese until melted.
Fold in the pasta, tuna and peas. Spread in the baking dish and top with the remaining cheese.
Melt the remaining tablespoon of butter and toss it with the breadcrumbs. Sprinkle over the top.
Bake for 20–25 minutes, until bubbly and golden.