Start by gathering all your ingredients. Make sure your leftover meatloaf is sliced and ready to go. If you want to add mayo or Greek yogurt, gather that as well.
Heat a skillet over medium heat. Lightly coat it with nonstick spray, canola oil, or butter to prevent sticking.
Take two slices of sourdough bread. Spread your buttery spread generously on one side of each slice. On the unbuttered side, layer your sliced meatloaf, cheese, sliced tomato, pickled onions, roasted red peppers, and arugula.
Carefully place the assembled sandwich in the heated skillet, buttered side down. Top with the other slice of bread, buttered side up. Cook for about 4-5 minutes or until golden brown.
Using your spatula, gently flip the sandwich over. Cook for another 4-5 minutes until the cheese has melted and the bread is golden brown.
Once cooked, remove the sandwich from the skillet. Let it sit for a minute, then slice it in half. Serve warm and enjoy your delicious Meatloaf Sandwich!
