In a medium skillet over medium heat, melt 2 tablespoons of butter. Add the diced apples, brown sugar, cinnamon, and nutmeg. Cook, stirring occasionally, until apples are softened and slightly caramelized, about 8-10 minutes. Set aside.

In a shallow dish, whisk together the eggs, milk, heavy cream, vanilla extract, and salt until well combined.

Slice croissants in half horizontally. Dip each half into the custard mixture, letting it soak for about 20-30 seconds per side.

In a large skillet or griddle over medium heat, melt the remaining 2 tablespoons of butter. Place the soaked croissant halves onto the hot skillet. Cook for 3-4 minutes per side, or until golden brown and cooked through.

Place cooked croissant French toast on plates. Top with the warm cinnamon apple mixture. Serve immediately.