Preheat oven to 375°.
Heat oil in a large nonstick skillet over medium heat. Add onion, and cook 5 minutes or until tender, stirring frequently. Add garlic and mushrooms; cook 8 minutes or until liquid evaporates, stirring frequently. Stir in meatless crumbles. Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, milk, salt, and pepper, stirring with a whisk. Add to pan. Bring to a simmer; cook 2 minutes. Place in an 11 x 7-inch baking dish. Top with corn.
Prepare potatoes according to package directions. Stir in sour cream and butter. Spread potatoes evenly over corn. Bake at 375° for 30 minutes or until browned.