1. Preheat grill to medium-high heat. Press 1 piece of cheese in center of each patty, covering cheese with meat to seal. Sprinkle burgers with salt and pepper.
2. Grill 5 to 6 minutes per side or until no longer pink. Grill onion slices 1 to 2 minutes per side.
3. Spread mustard on buns; serve burgers on buns with onion and spinach.
Side Dish Instructions
1. Preheat oven to 425°F. Combine potatoes, oil, salt, and pepper in a single layer on a rimmed baking sheet; toss.
2. Bake 20 to 25 minutes or until browned and tender.