Thinly slice tomato, cucumber, onion, and cheese. Heat a skillet to medium high.
Spread butter onto the outside of the bread slices. Top 4 of the slices of bread with tomato, cucumbers, onions, spinach, and cheese; add remaining bread slices, buttered-side out, forming 4 sandwiches.
Heat in skillet until cheese has melted, flipping to brown both sides.