Preheat oven to 200°F and line a large rimmed baking sheet with foil. Lightly mist a griddle or large skillet with cooking spray.
Use a 2-inch heart-shaped cookie cutter to cut three hearts from each slice of bread. Lightly brush both sides of bread with butter.
Place about 1 1/2 tsp. cheese in center of half of hearts. Use fingers to spread cheese and lightly pack it down, following shape of hearts. Top cheese with about 1/4 tsp. relish and season with pepper. Place remaining hearts on top of filling, lightly pressing down to form firm, well-shaped sandwiches.
Working in batches, cook sandwiches on griddle or in skillet over medium-high heat until lightly browned, 3 to 5 minutes per side. Use a small spatula to carefully flip sandwiches over. Place finished sandwiches on lined baking sheet and keep warm in oven until ready to serve.