Steps
1. Heat olive oil in a pan over medium heat.
2. Add the sliced onions and sauté until golden and caramelized, about 5-7 minutes.
3. Preheat your oven's broiler.
4. Place the slice of bread on a baking sheet.
5. Layer the sautéed onions on top of the bread, followed by the tomato slices.
6. Top with cheese and season with salt and pepper.
7. Broil in the oven for about 3-5 minutes, or until the cheese is melted and bubbly.
8. Remove from the oven, garnish with basil if desired, and serve warm.