Preheat oven to 190 C / Gas 5. Butter a medium baking dish with 1 tablespoon butter.
Bring a large pot of lightly salted water to the boil. Add pasta, cook for 8 to 10 minutes, until al dente, and drain.
Melt 1 tablespoon butter in a frying pan over medium-low heat. Stir in the onion, celery and garlic, and cook 5 minutes, until tender. Increase heat to medium-high, and mix in mushrooms. Continue to cook and stir 5 minutes, or until most of the liquid has evaporated.
Melt 4 tablespoons butter in a medium saucepan, and whisk in flour until smooth. Gradually whisk in milk, and continue cooking 5 minutes, until sauce is smooth and slightly thickened. Season with salt and pepper. Stir in tuna, peas, mushroom mixture and cooked pasta. Transfer to the baking dish. Melt remaining 2 tablespoons butter in a small bowl, mix with breadcrumbs, and sprinkle over the dish. Top with cheese.
Bake 25 minutes in the preheated oven, or until bubbly and lightly browned.
we used half a recipe as there are only 2 of us but we used a whole recipe of sauce, we also used sweetcorn instead of celery - for extra flavour we put half a vegetable stock cube in the water when cooking the pasta and the other half crumbled into the sauce - 22 Aug 2016