Cook macaroni as per package directions EXCEPT cook for about 2-3 minutes less so that the macaroni is slightly firm in the center. Drain and set aside.
Saute mushrooms in 1 tbs of the butter until mushrooms are brown and seared. Set aside
Preheat oven to 375 degrees F. Lightly spray or grease a 11 x 7-inch deep baking dish (about 3-inches high. If you don't have a deep 11 x 7, use a 13 x 9).
In a large (3-4 qt) saucepan or Dutch oven over medium heat, melt 3 tbs of the butter and add the minced garlic. Cook the garlic about 3 minutes or until it becomes aromatic. Do not brown the garlic.
Stir in the flour and cook for 1 minute.
Gradually add the milk while stirring to prevent lumps. Cook, stirring frequently until sauce begins to get thicker and begins to bubble.
Gradually add the cheeses, stirring until the cheeses are entirely melted.
Add the mushrooms and macaroni, stirring to blend well.
Place in prepared pan.
Melt the remaining 1 tbs of butter in a small saucepan. Add panko crumbs and toss to coat.
Sprinkle buttered crumbs on top of casserole.
Bake for 25-30 minutes, or until crumbs are golden brown. Let the casserole sit for 10 minutes before serving