Cut Four 1-inch thick slices of leftover meatloaf. Gently warm the slices in the microwave or toaster oven.
Heat a large non-stick pan over medium heat. Brush the olive oil on the cut side of the ciabatta rolls. Working in batches, place the buns in the pan and toast lightly. Set aside and repeat with the remaining bread.
To assemble the sandwich, spread the toasted rolls with the mayonnaise. Place several leaves of spinach on four of the ciabatta slices, followed by a slice of the meatloaf. Top with the remaining bread.