In sauté pan, add apple juice and onion slices; bring to a boil and simmer until no liquid remains. Turn the heat to medium-low, remove onions from pan, and set aside.
Lightly brush one side of each slice of bread with half the olive oil. Using the same pan used for the onions, turn up the burner to medium heat. Once the pan is hot, place one slice of bread oil side down into the pan. Quickly place cheese, bacon, and braised onions on bread in pan. Top with remaining slice, oil side up.
Once brown, flip the sandwich to toast other side. When the second slice is brown, place sandwich on serving plate. Rest for 5 minutes, cut in half and serve!