Preheat oven to 350°. Melt butter in Dutch oven. Sauté onions until just tender. Add shrimp and mushrooms and cook, stirring frequently, until shrimp are just done. Remove from pan (along with liquid) and drain in colander. Blend flour and mayonnaise in pan; add salt, milk, and sherry. Cook until thickened, stirring as you would any cream sauce. Toss with shrimp and spaghetti and turn into 3-quart casserole. Top generously with cheeses. Bake for 30 minutes.
Note: This can be fancied up further if you wish by adding a can or two of artichoke hearts, halved, or hearts of palm, sliced about 1/2 inch thick.
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