Step 1 Make the patties: In a large bowl, gently but thoroughly mix together the ground beef, Italian sausage, Italian seasoning, onion, bell pepper and 2 cloves minced garlic. Press the mixture into a jelly roll pan, using a rolling pin to make a flat, even layer. (Cover the meat with plastic wrap, if necessary, to keep it from sticking as it's rolled.) Cut the meat into roughly 2-inch squares.
Step 2 Heat a grill or grill pan over medium-high heat until hot. Grill the burgers until cooked to an internal temperature of at least 160 degrees, about 2 to 3 minutes per side. When the burgers are done, place a slice of mozzarella on each burger and tent with foil while preparing the buns.
Step 3Warm the rolls: Wrap the rolls in foil and place on the top rack of the grill, or place in a warm oven until warmed through.
Step 4In a small bowl, whisk together the olive oil, balsamic vinegar and remaining minced clove garlic. This makes a scant one-half cup vinaigrette.
Step 5Assemble the sliders: Place a cheese-topped patty on the base of the bun, add a slice of tomato and 2 basil leaves. Drizzle each burger with 1 teaspoon of the vinaigrette. Serve immediately.