Make the caramelized onions by heating the butter or olive oil in a small skillet over medium heat. Add the onion and cook, stirring often, for 20-25 minutes, or until golden brown. Season with a bit of sea salt, to taste. Side Note: Add a little water if the onions start to become dry.
Prepare the sandwich by spreading butter on one side of a piece of the French bread and placing it into a large skillet over medium heat.
Add a thin layer of Gruyere cheese on the bread, then spoon some of the slow-roasted cherry tomatoes and garlic on top, followed by a layer of caramelized onions.
Top with more Gruyere cheese slices, followed by the remaining piece of French bread.
Spread butter on the top of the bread and cook for 3-4 minutes, or until golden brown.
Carefully flip over and cook for an additional 3 minutes, or until golden brown.
Remove from the skillet and serve immediately. Enjoy!