In a large bowl, gently toss the tomato slices (being careful not to break them) with the olive oil, balsamic vinegar, salt and pepper until evenly coated. Lay the tomato slices in an even layer on a rimmed baking sheet. Scatter the garlic cloves and thyme (if using) evenly over the tomatoes.
Roast the tomatoes for 25 to 30 minutes or until shriveled and starting to brown at the edges.