Heat a large skillet over medium-high. Cook ground beef and onion until browned, about 8 minutes. Add garlic and cook until fragrant, about 1 minute.

Whisk together broth, vinegar, brown sugar, Worcestershire, salt, soy sauce, 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 on a baking sheet. Bake until crisp, 8 to 9 minutes.

Press a large spoon into the center of each toast to create a well. Spoon about 1/4 cup of meat mixture into each well. Top with cheese.

Return baking sheet to oven and bake until cheese is melted, about 5 minutes.

Serve immediately with pickle chips.