Heat a grill pan over medium-high heat. Combine first 5 ingredients in a medium bowl. Divide mixture into 4 equal portions, shaping each into a 1/2-inch-thick patty. Add patties to pan; cook 4 minutes on each side or until desired degree of doneness. Remove patties from pan. Add tomato slices to pan; cook 1 minute on each side. Spread about 3/4 teaspoon mustard over bottom half of each roll; top each with 1 lettuce leaf, 1 patty, 1 tomato slice, and top half of roll.