- 1 pound ground sirloin
- 1 pound ground chuck
- 1/2 cup chopped onion
- Kosher salt and freshly ground black pepper to taste
- 1 1/2 cups chopped fresh tomatoes
- 2 tablespoons chopped fresh basil
- 8 slices mozzarella cheese
- 8 hamburger buns, split
Preheat an outdoor grill for medium-high heat and lightly oil the grate.
Mix sirloin, chuck, onion, kosher salt, and pepper together in a bowl with your hands. Form meat mixture into eight 3/4-inch patties.
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Stir tomatoes and basil together in a bowl until well-combined.
Cook patties on the preheated grill until they start to firm, and are reddish-pink and juicy in the center, 3 to 4 minutes per side. An instant-read thermometer inserted into the center should read 130 degrees F (54 degrees C). Place a slice of mozzarella cheese onto each burger during the last minute of cooking.
Place 1 burger on the bottom half of each bun, spoon some tomato mixture onto each burger, and place top bun on top.