- 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.
- 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.
- 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.
- Divide the onion mixture between four slices of bread. Top with the tomatoes, arugula and remaining cheese. Close the sandwiches.
- 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.