Gather ingredients. Preheat oven to 425°F (220°C).

Heat a large skillet over medium-high. Add ground beef and onion; cook, stirring often to crumble, until meat is browned and onion is softened, about 8 minutes. Add garlic and cook, stirring constantly, until fragrant, about 1 minute.

While meat cooks, whisk together broth, vinegar, brown sugar, Worcestershire sauce, salt, soy sauce, yellow mustard, and pepper in a small bowl.

Stir broth mixture into skillet and cook, stirring often, until sauce has reduced slightly, about 2 minutes. Remove from heat.

Arrange toasts in an even layer on a large rimmed baking sheet. Bake in the preheated oven until crisp and lightly browned on the bottoms, but still soft in the center, 8 to 9 minutes.

Use the round side of a large spoon to press into the center of each toast to create a well, leaving about a 1/4-inch border around the edge of each bread. Spoon about a heaping 1/4 cup of meat mixture into each well. Sprinkle meat and toasts with cheese (about 1/4 cup per toast).

Return baking sheet to oven and bake at 425°F (220°C) until cheese is just melted and toasts are crisp, about 5 minutes.