Melt butter in a Dutch oven over medium heat; add garlic, and sauté until tender. Add milk, and cook until thoroughly heated. Gradually stir in 1/2 cup Parmesan cheese and next 6 ingredients until smooth. Stir in spinach, if desired. Add pasta, tossing to coat; spoon into a lightly greased 13- x 9-inch baking dish.
Bake at 400° for 7 minutes. Remove from oven; sprinkle with remaining 1/2 cup Parmesan cheese and breadcrumbs. Reduce oven temperature to 350°.
Bake at 350° for 20 minutes.