- Stir together ingredients for barbecue sauce in a saucepan over medium heat. Bring to a simmer and reduce heat to medium-low. Simmer for 15-20 minutes, until sauce has thickened. Pour into a small bowl and cool for 10 minutes.
- Brush pineapple slices with 2 tablespoons of barbecue sauce and grill over medium-high heat until grill marks appear, about 3 minutes each side.
- Place 2 pieces of bread on a clean work surface. Top each piece of bread with 2 slices of cheese, 2 slices of pineapple and 3 slices of red onion. Place remaining slices of bread on top. Heat 1 tablespoon of butter in a non-stick skillet over medium heat. Once butter melts, add one sandwich and cook for 3 minutes on each side. Repeat with remaining sandwich and butter.
- Serve warm with remaining barbecue sauce for dipping.