Roast Beef and French Onion Grilled Cheese

Heat olive oil in a medium skillet over medium heat. Add onion, season with 1/4 teaspoon each salt and pepper, and cook, covered, stirring occasionally, for 12 minutes. Reduce the heat to medium-low, stir in thyme leaves and cook, uncovered, stirring occasionally, until the onions are golden brown, 15 to 20 minutes more (add 1 tablespoon water to the skillet if the onions start sticking). Brush one side of each slice of bread with butter or oil. Form sandwiches with the bread, whole-grain mustard, Gruyère cheese, roast beef, and the onion mixture. Cook the sandwiches, covered, until the bread is golden brown and crisp and the cheese has melted, 4 to 5 minutes per side.