Ingredients Meat & Poultry Beef Recipes Ground Beef Recipes Open-Faced Cheeseburgers with Mushrooms and Onions 3.0 (4) Add your rating & review By Martha Stewart Test Kitchen Martha Stewart Test Kitchen The recipes developed by our test kitchen team have undergone a rigorous process of development and testing, ensuring that every element is optimal, from ingredient amounts to method and cooking time. This process includes triple-testing recipes to ensure they meet our high standards. The many stellar cooks and food editors who have been part of our team include Sarah Carey, Lucinda Scala Quinn, Jennifer Aaronson, Shira Bocar, Anna Kovel, Greg Lofts, Riley Wofford, Lauren Tyrell, and Lindsay Leopold. Editorial Guidelines Updated on May 16, 2017 Print Rate It Share Share Tweet Pin Email Prep Time: 10 mins Total Time: 25 mins Servings: 4 Fontina is especially good on this burger. When heated, its texture becomes remarkably creamy and luscious. Ingredients 2 tablespoons olive oil 10 ounces white mushrooms, trimmed and thinly sliced Coarse salt and ground pepper 1 medium red onion, halved and thinly sliced 1 teaspoon red-wine vinegar 1 pound ground beef chuck 1 plum tomato, cored and cut lengthwise into 4 slices 2 ounces Fontina cheese, cut into 4 slices 4 slices sourdough bread (1 inch thick), toasted Directions In a large skillet, heat 1 tablespoon oil over medium-high. Add mushrooms; season with salt and pepper. Cook, stirring occasionally, until mushrooms release their liquid and start to brown, 5 to 7 minutes; transfer to a bowl (reserve skillet). Add onion and remaining tablespoon oil to skillet; season with salt and pepper. Cook, stirring occasionally, until onion starts to brown, 5 to 7 minutes (if bottom of skillet becomes too dry, add a little water and scrape up browned bits). Return mushrooms to skillet; stir in vinegar and set aside. Heat broiler, with rack set 4 inches from heat. Gently form meat into 4 3/4-inch-thick patties; season with salt and pepper. Place on a rimmed baking sheet, and broil 3 minutes. Remove baking sheet; top burgers with tomato and cheese slices. Broil until cheese is melted and burgers are medium-rare, 1 to 2 minutes more. Serve burgers on toasted bread, with sauteed mushrooms and onion alongside. Cook's Notes If you prefer burgers that are done medium instead of medium-rare, broil the patties for 1 to 2 minutes longer before topping them with tomatoes and cheese. Rate it Print