01 - Cook bacon in a skillet over medium heat until crisp. Drain on paper towels. While warm, brush both sides lightly with maple syrup.
02 - Lightly butter one side of each slice of bread.
03 - On the unbuttered side of two bread slices, layer cheddar cheese, brie cheese, prepared maple bacon, and apple slices (if using). Garnish with fresh thyme if desired.
04 - Spread a thin layer of Dijon mustard on the unbuttered side of the remaining bread slices (if using). Invert these slices onto the assembled sandwiches, ensuring the buttered side faces outward.
05 - Heat a large skillet or griddle over medium-low heat. Place the assembled sandwiches in the skillet and cook for 3–4 minutes per side. Press gently throughout cooking until the bread is golden brown and the cheese is fully melted.
06 - Slice the grilled cheese sandwiches in half and serve immediately.