A new twist on a favorite classic soup. This casserole is filled with tender beef, caramelized onions, mashed potatoes and mushrooms for a hearty, comforting meal.

French Onion Beef Casserole

Preheat oven to 375 degrees. Butter a 13x9x2 baking dish with 1 Tbsp. butter; set aside. Microwave mashed potatoes according to pkg. directions. Spread potatoes in bottom of prepared baking dish; set aside.

Spread butter on both sides of baguette slices, using 2 Tbsp. Place in a large rimmed nonstick baking pan. Bake for 10 to 12 minutes or until lightly golden, turning bread slices halfway through; set aside.

Place oven rack 6 in. from heat. Preheat broiler to HIGH. Top toasted slices with 1 cup cheese. Broil for 2 to 3 minutes or until cheese is bubbly and begins to brown; set aside.

Meanwhile, melt remaining 3 Tbsp. butter in a 6-qt. Dutch oven. Add onions; cook over medium to medium-low heat for 20 to 30 minutes or until onions begin to soften and turn brown, stirring often. Add 2 Tbsp. thyme and pepper; cook for 20 to 30 minutes more or until onions are caramelized and golden brown, stirring often. Stir in flour. Cook and stir for 3 minutes. Transfer onions to a bowl; set aside.

Pat beef dry with paper towels. Cook beef in Dutch oven over high heat for 4 to 6 minutes or until beef begins to brown. Add broth, mushrooms, sherry vinegar and caramelized onions; cook for 2 to 3 minutes or until combined, scrapping bits from bottom of Dutch oven. Slightly cool mixture.

Spread onion-beef mixture evenly on top of potatoes in baking dish; sprinkle with remaining ½ cup cheese. Place cheese toasts on top. Cover with foil.

Bake for 15 to 20 minutes or until heated through. Cool slightly before serving. Garnish with additional thyme and black pepper, if desired.