Preheat the oven to 350°F Grease a 2-quart casserole dish.
Melt 4 T of butter over med heat in a large saucepan. Add the mushrooms, bell pepper, and onion and saute until the mushrooms give up their juice, about 4 minutes. Stir in the flour until it forms a paste. Stir in the milk, increase the heat, and bring to a boil. Remove from the heat and stir in the tuna, breaking up any clumps. Season to taste with salt and pepper.
Add the noodles and peas tot he sauce and mix well. Taste and adjust the seasoning as needed.
Transfer the mixture to the casserole dish and sprinkle the top with the bread crumbs. Melt the remaining 2 T butter and drizzle over the casserole.
Bake for 25 to 35 minutes, until the topping is browned. Serve hot.