Chicken Tetrazzini Casserole

1/2 lb. spaghetti noodles, cooked
1/2 pound fresh sliced mushrooms
1 stick butter
2/3 cup flour
1 large onion, sliced
1 large red bell pepper, halved and sliced very thin
5 cloves garlic, peeled and chopped
4 c. chicken broth, hot
2 c. light cream or whole milk, hot
1/4 cup dry sherry
4 c. diced cooked chicken
2 c. grated Parmesan cheese
1/3 teaspoon garlic powder
Salt and pepper to taste
Cook and drain pasta according to package directions. Slice the mushrooms 1/8 of an inch thick and saute in a single layer in 2-3 tablespoons of the butter until nicely browned on both sides. Remove from pan and lightly brown the sliced onion and red bell pepper, about 10 minutes or so. Add thin sliced or chopped garlic just for the last 1 or 2 minutes so that it does not brown and become bitter. Remove to large buttered casserole dish. In pan, melt the remaining butter until it foams then sprinkle with flour and whisk continually over med low heat to keep from scorching. When it begins to smell nutty instead of like raw flour continue whisking briskly while adding hot milk then the chicken broth in steady stream. Stir occasionally as it thickens, maybe 10 minutes. Finally add salt, pepper,and sherry. And the chicken and 1-1/2 cups of the parmesan and turn into casserole with vegetables. Mix in spaghetti. Mixture should be fairly creamy and look like there is a little too much sauce. It thickens more during cooking. Top with 1/2 c parmesan and bake uncovered at 400 degrees for 20-30 minutes till browned and bubbly.