Preheat a large nonstick skillet over medium-low heat. Lightly coat the skillet with nonstick spray or brush with canola oil or additional butter.

While the skillet heats, spread a thin layer of mayonnaise or plain Greek yogurt (use the 2–3 tablespoons as desired) on one side of each of the 8 slices of bread. Spread the 1/4 cup buttery spread (or softened butter) in a thin layer on the opposite side of each slice, spreading to the edges.

Place 2 slices of meatloaf in the hot skillet. Cook until the meatloaf is warmed through and lightly browned in places, about 3 minutes per side. Remove the warmed meatloaf slices to a plate.

Lay 2 slices of the prepared bread in the skillet, buttered-sides down. Place 1 slice of provolone cheese on each bottom slice of bread. Top each cheese with one warmed meatloaf slice and your desired toppings (sliced tomato, pickled onions, roasted red peppers, and arugula).

Place the remaining 2 bread slices on top of the sandwiches, buttered-sides up. Grill the sandwiches in the skillet over medium-low heat until the bread is toasted and the cheese is melted, about 2 to 4 minutes per side. Flip carefully and press gently with a spatula if desired.

Transfer the cooked sandwiches to a cutting board and let them rest for a few minutes before serving.

Repeat steps 3–6 with the remaining 2 slices of meatloaf, 2 slices of cheese, and the remaining 4 slices of bread to make the second pair of sandwiches.