Heat oven to 375°F. Prepare Horseradish Butter; set aside. Melt 2 teaspoons butter in large nonstick skillet over medium heat. Add onion and thyme; cook and stir 3 to 4 minutes or until onion is tender. Remove from heat; stir in Corned Beef and horseradish-butter mixture. Meanwhile, coat bottom and sides of 9-inch glass pie plate with remaining 1 teaspoon butter; set aside.
Arrange 1/2 of potato wedges in single layer over bottom of pie plate; stick remaining potato wedges upright around the edge of the pie plate. Combine cheeses in small bowl. Sprinkle potatoes with 1/2 of cheese mixture. Top with corned beef mixture and remaining cheese mixture. Press firmly with spatula to compact layers; cover with aluminum foil.
Bake in 375°F oven 25 to 30 minutes or until heated through. Uncover; continue baking 3 to 5 minutes or until cheese is melted and edges begin to brown. Let stand 5 minutes. Cut into wedges.
Nutrition information per serving: 445 Calories; 252 kcal Calories from fat; 28 g Total Fat; 13 g Saturated Fat; 11 g Monounsaturated Fat; 121 mg Cholesterol; 1130 mg Sodium; 121 g Total Carbohydrate; 1.5 g Dietary Fiber; 27 g Protein; 2.5 mg Iron; 2.6 mg Niacin; 0.3 mg Vitamin B6; 2.3 mcg Vitamin B12; 5.1 mg Zinc; 33 mcg Selenium;