-
1
1. In a nonstick skillet over medium-high heat, combine the butter and olive oil, heating until the butter has melted. Add the onion slices and saute, stirring until they start to brown, about 10 minutes. Season with salt and pepper.
-
2
2. Add a splash of water to deglaze the pan, then lower the heat to medium and continue cooking until the onions are soft and caramelized, about another 15 minutes stirring often. If the pan gets dry, add a bit more water.
-
3
3. Standing back from the pan, stir in the balsamic vinegar and remove from the heat. Let the onions cool to room temperature. The onions can be refrigerated for later use. If so, let warm at room temperature for 30 minutes before adding to the sandwich.
-
4
4. Meanwhile, in a small bowl whisk together the mayonnaise, mustard, garlic and lemon juice. Season with salt and pepper, then set aside. This also can be prepared ahead and refrigerated.
-
5
5. To assemble the sandwiches, lightly toast the bread. Let the bread cool for a minute, then spread some of the mayonnaise mixture on all 4 slices.
-
6
6. Top 2 of the bread slices each with a slice of meatloaf. Place half of the onions over each meatloaf slice, then top each sandwich with 2 slices of cheese, a leaf of lettuce and a final slice of bread. If desired, trim the ingredients to fit neatly between the bread slices. Cut in half on the diagonal and serv
