Tomato and Caramelized Onion Grilled Cheese Sandwich

Preparation

1. On a plate, lay out the tomato slices. Season with salt to extract any excess water. Let sit while you prepare the remaining ingredients. Pat dry with paper towel.
2. In a non-stick skillet over medium-low heat, cook the onions in 1 tbsp (15 ml) of the butter until translucent. Cover and cook over low heat for 10 minutes. Remove the cover and continue to cook over medium-low heat, stirring constantly, until caramelized, about 2 minutes. Let cool. Stir in the mustard.
3. On a work surface, butter one side of each bread slice with the remaining butter. Sprinkle 1 cup (100 g) of the cheese over the buttered sides of the bread, pressing to adhere. Turn the bread slices over, cheese side down.
4. Divide the onion mixture between four slices of bread. Top with the tomatoes, arugula and remaining cheese. Close the sandwiches.
5. In the same skillet over medium-low heat, brown two sandwiches at a time for 2 minutes on each side, pressing them down with a spatula. Cut each sandwich in half and serve immediately.