Garlic-Thyme Burger with Grilled Tomato

• 1 Tbsp chopped fresh thyme
• 3/8 tsp kosher salt
• 1/4 tsp freshly ground black pepper
• 2 garlic cloves, minced
• 1 lb ground sirloin
• 4 (1/2-inch-thick) slices beefsteak tomato
• 1 Tbsp Dijon mustard
• 4 (2-ounce) Kaiser rolls or other sandwich rolls
• 4 baby romaine lettuce leaves

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.