Preheat oven to 375°.
Cook pasta according to the package directions, omitting salt and fat. Drain well. Set cooked pasta aside.
Melt butter in a large nonstick skillet over medium-high heat. Add shallots; sauté 3 minutes. Add mushrooms, 1 tablespoon thyme, salt, pepper, and garlic; sauté 8 minutes or until mushrooms are tender. Add sherry; cook 1 minute, stirring frequently. Remove from heat.
Place flour in a Dutch oven over medium-high heat; gradually add milk, stirring constantly with a whisk. Bring mixture to a boil; cook 1 minute or until slightly thick, stirring constantly with a whisk. Remove from heat; add 1/2 cup cheese, stirring until melted. Add pasta and mushroom mixture to cheese mixture, tossing well to combine. Spoon pasta mixture into an 8-inch square baking dish lightly coated with cooking spray; sprinkle evenly with remaining 1/2 cup cheese. Bake at 375° for 30 minutes or until cheese melts and begins to brown. Garnish with thyme sprigs, if desired.