Coat a large, nonstick skillet with nonstick spray, or brush with oil or butter so that you have a light coating. Heat the skillet over medium low. Place two slices of meatloaf on the skillet. Cook on each side until the meatloaf is lightly browned in places and warmed through, about 3 minutes per side. If your bread is larger than the meatloaf slices, you can grill a few extra slices, then piece a few together for each sandwich (the cheese will hold the pieces together).
Meanwhile, spread one side of each slice of bread with a thin layer of mayo (or Greek yogurt) and the other with a thin layer of buttery spread, spreading the butter all the way to the edges. Remove the meatloaf slices from the skillet. Lay 2 slices of bread in the skillet, buttered-sides down. Top each with a slice of cheese, a slice of warmed meatloaf, and any other desired toppings. Lay another slice of bread on top, buttered-sides up.
Grill the meatloaf sandwiches on each side until the bread is toasted and the cheese is melted, about 2 to 4 minutes per side (be patient! Low-and-slow guarantees melted cheese and crisp, but not burned bread). Repeat with the remaining two sandwiches. Let sandwiches cool for a few minutes before serving.
Notes
TO STORE: Refrigerate leftover meatloaf sandwich in an airtight storage container for up to 1 day.
TO REHEAT: Rewarm leftovers in a skillet on the stovetop over medium-low heat or with a panini press, or enjoy sandwich cold.
TO FREEZE: Wrap individually in plastic wrap and then foil. Let thaw overnight in refrigerator before reheating.