01 - Cook bacon in a skillet over medium heat until crispy. Drain on paper towels and brush both sides lightly with maple syrup while still warm.
02 - Spread a thin layer of softened butter on one side of each slice of bread.
03 - On the unbuttered side of two bread slices, layer cheddar cheese, brie cheese, maple-coated bacon, and thin apple slices if desired. Sprinkle with fresh thyme leaves if using.
04 - Spread Dijon mustard on the unbuttered side of the remaining bread slices if desired, then place them on top of the sandwiches, buttered side facing outward.
05 - Heat a large skillet or griddle over medium-low heat. Place the assembled sandwiches in the skillet and cook for 3 to 4 minutes per side, gently pressing until the bread is golden brown and the cheese is melted.
06 - Slice sandwiches in half and serve immediately while warm.