Step 1 Heat the oven to 375 degrees. Brush a 9-inch round baking dish with 2 teaspoons of the butter.
Step 2 Spray a saucepan with nonstick cooking spray. Add 1 tablespoon of the butter and heat until the butter is melted. Add the onion and celery and cook over medium heat until the vegetables are tender, about 3 minutes. Stir in the mushrooms and continue to cook, stirring, until the mushrooms are tender, about 2 minutes.
Step 3Remove the saucepan from the heat and stir in the flour until blended. Gradually blend in a little milk and stir until the mixture looks smooth. Stir in the remaining milk and half-and-half. Heat over medium heat, stirring, until the sauce begins to simmer and thicken, 1 to 2 minutes. Stir in the peas. Season with the salt and pepper.
Step 4Cook the noodles in boiling salted water until they are tender, 5 to 7 minutes. Drain.
Step 5Spread 1/3 of the drained noodles evenly across the baking dish. Arrange 1/2 of the drained tuna over the noodles, then pour 1/3 of the sauce over the top, spreading to the edges. Repeat the layers. Top with the remaining noodles and sauce.
Step 6Heat the remaining 1 teaspoon of butter in a small nonstick skillet over medium-low heat until the butter is melted. Add the bread crumbs and stir until the bread crumbs begin to brown, about 2 to 3 minutes. Spoon the bread crumbs over the top of the casserole.
Step 7Bake until the casserole is hot through the center, 30 to 35 minutes.