Step 1Spread butter on each slice of bread, 1 side only. Cover unbuttered side with 1/2 cheese. Top with Ratatouille, keeping it slightly away from edge of bread. Cover with remaining cheese. Close sandwich, buttered side up.
Step 2Place sandwich in skillet over medium-low heat. Cook, covered, until browned, about 4 minutes. Gently compress sandwich. Turn. Brown other side, covered, about 4 more minutes. Remove from heat. Let rest 10 minutes. Use serrated knife to cut in half. It's best when hot.