"I've had this recipe for quite some time, but I'm not sure where it came from! This is a simple, easy, burger and absolutely delicious! Also, I do not always cook on a grill, and this is just as wonderful if you don't!"
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.
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.