- Add sliced onions to a 16-ounce glass jar or bowl. Combine hot water, sugar and sea salt in a measuring cup with pour spout and stir until sugar is dissolved. Add rice vinegar. Pour mixture over red onion. Press onions down so they are completely submerged. Cover with lid and set out at room temperature for one hour.
- After onions are ready, spread butter on one side of four slices of bread. Place buttered side of bread facing down and top with 2 slices of cheese, 2 slices of apple, ¼ cup pickled onions and 2 slices of cheese. Place the other piece of bread on top, buttered side facing up. Repeat with remaining bread.
- Heat a grill pan over medium heat and spray with cooking oil. All pan to heat for 3 minutes. Place 1 sandwich on pan and cook for 3-4 minutes. Gently flip using a spatula and grill for 3-4 minutes longer. Repeat with the remaining sandwich. Slice sandwiches in half and enjoy warm.