Preheat oven to 375° Fahrenheit.
In large, deep saucepan, melt 1/2 tablespoon butter until sizzling. Add onion; sauté over medium heat about 3 minutes or until tender. Add remaining butter. When sizzling add the flour and cook, stirring, until the flour is golden.
Over medium heat, gradually whisk milk and chicken broth into flour butter mixture. Cook, stirring constantly 8 to 10 minutes, until the mixture is thick and smooth. Stir in tomatoes, rosemary, salt and pepper. Bring just to boiling point and remove from heat. Add 2/3 cup each of the Cheddar and Monterey Jack cheeses. Stir until the cheese is melted. Stir in crabmeat and sherry. Stir in the pasta, mixing well.
Transfer mixture to an oval 8" x 10" lightly buttered casserole (or slightly larger).
Sprinkle the remaining cheese over the casserole and bake until bubbly, about 20 minutes.