Melt butter in a medium saucepan over medium heat. Once melted, add the diced apples and stir to coat evenly. Cook for about 2 minutes until apples begin to soften slightly.
Add the brown sugar, cinnamon, nutmeg, and salt to the apples. Stir well to ensure even coating, then reduce heat to medium-low and simmer for 5-7 minutes until apples are tender but still hold their shape.
Remove from heat and stir in lemon juice and vanilla extract. Set aside to cool slightly while you prepare the custard.
In a wide, shallow bowl, whisk together eggs, milk, sugar, vanilla, cinnamon, and salt until completely combined.
Lay out your bread slices and divide the cooled apple filling among four slices, using approximately 3-4 tablespoons per sandwich. Spread the filling evenly, leaving about a half-inch border around the edges.
Top with remaining bread slices to form four sandwiches, pressing gently around edges to seal.
Heat a large skillet or griddle over medium heat and add butter. While the surface heats, carefully dip each sandwich into the egg mixture, allowing it to soak for about 20 seconds per side.
Transfer soaked sandwiches to the hot skillet and cook for 3-4 minutes per side until golden brown and cooked through.
Transfer to plates, dust with powdered sugar, and drizzle with warm maple syrup. Top with whipped cream, fresh apple slices, and chopped toasted pecans if desired.