In a large skillet, poach fish in water for 12-15 minutes, very gently until fish flakes. Drain. Break fish into large chunks and set aside. In a large saucepan, cook onion in 2 tablespoons butter until tender. Blend in salt and pepper. Add milk all at once. Cook, stirring constantly, until mixture thickens. Add cheese, sour cream, mushrooms, fish and peas. Gently stir and pour into 2 quart casserole dish. Combine crumbs and melted butter. Sprinkle on top. Bake 375 degrees for ½ hour.