Place the 2 teaspoons of oil into a skillet over medium heat. Add the sliced onions and cook, stirring often, for 20 minutes, or until caramelized and golden brown. Season with sea salt, to taste. Side Note: If the skillet gets dry, add a tablespoon of water or two. Remove from the skillet and set aside.
Preheat the oven to 350 degrees. Line a baking sheet with tin foil.
While the oven is preheating, heat the same skillet over medium heat and coat with cooking spray. Add the tomatoes and season with sea salt and freshly cracked pepper, to taste. Cook, stirring occasionally, for 5-7 minutes or until the tomatoes are softened and starting to burst. Remove from heat.
Spread the mayonnaise and Dijon mustard on each piece of artisan bread. Layer the bread with the caramelized onions, tomatoes, ham, and Swiss cheese. Place on the baking sheet and place into the oven for 5 minutes or until the cheese has melted. Remove from the oven and serve immediately. Enjoy.