Preheat a panini press according to the manufacturer's instructions. If using a skillet, heat a large skillet over medium heat and keep a heavy pot or second skillet nearby to use as a weight.

Spread the Dijon mustard evenly over 4 slices of bread. Then spread the cranberry sauce evenly over the remaining 4 slices.

Layer 2 slices of cheddar cheese, a portion of the sliced turkey, and a few baby spinach leaves onto the mustard-spread bread slices. Top with the cranberry-spread bread slices, cranberry-side down, to form 4 complete sandwiches.

Butter the outside of each sandwich generously on both sides with the softened butter.

If using a panini press, place the sandwiches inside and close the lid, pressing down gently. Cook for 4 to 6 minutes until the bread is deeply golden brown and crisp with clear grill marks, and the cheese is fully melted.

If using a skillet, place one buttered sandwich in the hot skillet and set a heavy pot or second skillet directly on top to press it down. Cook for 3 to 4 minutes per side, pressing occasionally, until golden brown and crisp on both sides and the cheese has melted throughout.

Transfer the cooked panini to a cutting board and let rest for 1 to 2 minutes before slicing in half on a diagonal.

Serve warm and enjoy this fast, satisfying way to turn Thanksgiving turkey leftovers into a crispy, flavorful sandwich.