Divide beef into 4 equal portions; shape into patties. Flatten each patty to a 1/4-inch thickness. Sprinkle evenly with 1/4 teaspoon salt and 1/4 teaspoon pepper.
Heat a large cast-iron skillet over medium-high heat. Add 2 teaspoons oil to pan; swirl to coat. Add patties to pan; cook 2 minutes. Turn patties; top evenly with mozzarella cheese. Cook 2 minutes or until cheese melts. Remove patties from pan. Coat bread slices with cooking spray. Add 4 bread slices to pan; cook 30 seconds on each side or until toasted. Repeat procedure with remaining bread slices and cooking spray. Cut 1 garlic clove in half; rub cut sides of garlic on both sides of bread slices.
Mince remaining garlic clove. Combine minced garlic, remaining 1/4 teaspoon salt, remaining 1/4 teaspoon pepper, remaining 1 teaspoon oil, tomato, and basil in a bowl; toss to coat. Top each of 4 bread slices with 1 patty. Divide tomato mixture evenly among patties. Top with remaining 4 bread slices.