Spray a 9x13 inch casserole dish with nonstick cooking spray.
Place 8 bread slices in prepared dish, slightly overlapping slices. If necessary, cut bread to fill in any empty spaces.
In a large skillet, over medium heat, melt 2 tablespoons butter. Add apples and cook just until tender, about 8-10 minutes. Remove from heat and add lemon juice and 1-teaspoon apple pie spice.
Spread apple mixture over bottom layer of bread, then top with remaining bread slices.
In a large bowl, whisk together eggs, milk, granulated sugar, vanilla extract and salt until well combined.
Pour the egg mixture over the bread and press down gently with the back of a wooden spoon or a rubber spatula. Cover and refrigerate overnight or at least 8 hours.
When ready to cook, preheat oven to 350 degrees F. Transfer French Toast to countertop and let stand at room temperature for 30 minutes while oven preheats.
In a small skillet or a microwave-safe bowl, melt 4 tablespoons butter and mix with ¼ teaspoon apple pie spice. Drizzle the apple spice butter over the bread.
Bake French toast until center is set and is golden brown, 40-50 minutes. Remove from oven and let stand 10 minutes before serving.
To serve, sprinkle with confectioner’s sugar. The French Toast is rich, but if desired, drizzle with maple syrup.