In a large sauté pan, heat the avocado oil over medium heat. Add the onions, sel gris, sugar, and Mediterranean rub.
Cook over medium-low heat, stirring occasionally, as the onions slowly caramelize and reduce, about 50-60 minutes
Turn the heat up to medium high and add the wine to deglaze the pan. Stir in the sherry vinegar. Set aside.
Preheat the oven to 400°F.
Cut the baguette into ¼” slices and arrange on a baking sheet. Drizzle with rosemary olive oil and bake until light brown in color, about 10 minutes.
Top each of the baguette slices with mozzarella cheese, caramelized onions, and a pinch of Mediterranean rub. Return to the oven until the cheese is melted and golden in color, about 10 minutes. Enjoy!