It is always interesting to watch onions, full of tear-jerking sulfur in raw form, slowly caramelize and transform into silky strands that are sweet in both texture and flavor. As long as you have 40 minutes to prepare the onions (with only a few minutes of actual work), you will be left with a great filler for sandwiches, omelettes, soups, pizzas and pies. Here, a classic pairing with gruyère cheese, which can be substituted with swiss if necessary.
Preheat the broiler. Heat the olive oil in a small skillet over medium heat. Add the onions, sprinkle with pepper and a generous amount of salt, and add the bouquet garni and garlic clove. Sautée until onions are slightly softened, about 5 minutes. Then turn the heat down to low and cook onions, stirring occasionally, until golden and softened, about 40 minutes. Remove from heat and discard the bouquet garni and the garlic.
Spread the onions over one bread slice. Top with sun-dried tomatoes. Rub remaining sandwich slice with the olive tapenade (if using). Sprinkle the gruyère on top of sun-dried tomatoes, place both slices in the broiler with ingredients facing up, and broil until cheese is bubbly and slightly browned, about 5 minutes.
Place sandwich slices together with ingredients facing in.