Tuna Tetrazzini

8 ounces thin spaghetti, cooked al dente and drained6 tablespoons butter, divided16 ounces thinly sliced mushrooms1 large onion, sliced4 Tablespoons all-purpose flour2 cups milk1/2 teaspoon salt, or to tastefreshly ground black pepper to taste2 cans (4 to 6 ounces each) tuna, drained and flaked1 cup frozen green peas, cooked and drained1 cup shredded natural Parmesan cheese, or about 1/2 cup finely grated Parmesan cheese1 cup bread crumbsDirections:
Heat oven to 350°F. Grease a 2 1/2- to 3-quart baking dish. In a large saucepan over medium-low heat, melt 4 Tablespoons of the butter. Add the mushrooms and saute, stirring, until mushrooms are browned and tender. Add the chopped onion and continue cooking for 1 minute. Stir in the flour until well blended. Gradually stir in the milk. Continue cooking, stirring constantly, until thickened. Add salt, to taste, and pepper. Stir in the flaked tuna and peas. Stir in the spaghetti until well blended. Turn the mixture into the prepared casserole. Sprinkle Parmesan over the casserole. Melt the remaining 2 Tablespoons of butter and toss with the bread crumbs. Sprinkle the buttered crumbs over the Parmesan layer. Bake for 25 to 30 minutes, until lightly browned and bubbly. Serves 6.