Cut apple into small cubes and combine with a splash of calvados. Toss leek slices with a pinch of nutmeg and salt. Whisk egg with salt and pepper. Pound schnitzel lightly to flatten slightly, then trim any ragged edges.
Arrange cheese strips, ham, bacon, apple cubes, and leek slices on one half of the schnitzel, leaving 1 cm clear around the edges. Season the filling with pepper.
Fold schnitzel in half and secure with toothpicks. Dust all sides with flour, dip in egg mixture, then coat with breadcrumbs, pressing gently so they adhere.
Heat clarified butter in a large pan over medium-high heat. Fry schnitzel for 3 minutes per side until golden and cooked through (internal temperature 63°C / 145°F), continuously drizzling hot butter over the top as it cooks.
Optional: Mix any leftover filling with remaining whisked egg and fry as scrambled eggs in a corner of the pan for about 2 minutes.