Melt 1 tbsp butter in a large nonstick skillet over medium heat. Add the apple slices, 1 tsp cinnamon, brown sugar, lemon juice, and a pinch of salt. Cook, stirring occasionally, until softened and glossy, about 5 minutes. Transfer to a bowl and wipe the skillet clean.

In a shallow dish, whisk together the eggs, milk, vanilla, remaining ½ tsp cinnamon, granulated sugar, and a pinch of salt until smooth.

Melt the remaining 1 tbsp butter in the same skillet over medium heat. Dip each bread slice into the custard for 15 to 20 seconds per side, letting excess drip off. Cook for 2 to 3 minutes per side until deep golden and crisp at the edges. Work in batches if needed.

Stack the French toast on plates, spoon the warm apples and pan juices over the top, and serve right away.