Heat oven to 350 F.
Grease a 2 1/2 to 3-quart baking dish.
Cook the spaghetti following the package directions in salted water. Drain and set aside.
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 green onions 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.
Combine the tuna sauce mixture with the drained spaghetti until well blended. Turn the mixture into the prepared casserole. Sprinkle Parmesan cheese 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 in the preheated oven for 25 to 30 minutes, until lightly browned and bubbly.