Cheesy Beef Stuffed Tomatoes

Heat the broiler and position the rack 6 inches below the heat source. Line a large rimmed baking sheet with foil. Heat the oil in a large skillet over medium heat. Add the onion, season with 1/4 teaspoon each salt and pepper, and cook, stirring occasionally, until tender and beginning to turn golden brown, 8 to 10 minutes. Stir in the thyme and garlic and cook for 1 minute. Add the beef, season with 1/2 teaspoon each salt and pepper and cook, breaking it up with a spoon, until browned, 6 to 8 minutes. Add the wine and simmer, stirring occasionally, until nearly evaporated, 3 minutes. While the onions are cooking, using a sharp knife and working on an angle, remove the tomato stem, leaving a 2-inch-wide opening in the tomato. With a spoon, scoop out most of the seeds and pulp (without breaking the skin). Transfer the tomatoes to the prepared baking sheet. Divide the meat mixture among the tomatoes (about 1/2 cup each) and top with the cheese (about 2 tablespoons each). Broil until the cheese melts and is beginning to brown, 2 to 3 minutes. Serve with a salad, if desired.