Remove the skin from the veal liver and rinse under cold water. Cut out any visible veins. Cut the liver into 20 slices, each approximately 80g.
Cut the onions into thin julienne (matchstick cuts). Place the apple slices in a bowl of cold water with lemon juice to prevent browning.
Lightly flour the onion julienne. Heat the clarified butter in a large pan over medium-high heat. Fry for 5-7 minutes, stirring occasionally, until golden brown and crispy. Drain on paper towels and set aside.
In a clean pan, heat the butter over medium heat. Add the drained apple slices and cook for 3-4 minutes on each side until tender. Sprinkle with sugar and cook for 1-2 minutes more until lightly caramelized. Set aside.
Season the liver slices with salt and pepper. Lightly flour the slices. Heat butter in a large pan over medium-high heat. Fry for 2-3 minutes on each side until the centers show a slight blush of pink.
Arrange the cooked liver on a serving platter. Drizzle with the butter from the cooking pan. Top each liver slice with a caramelized apple slice. Garnish with the crispy fried onions. Serve with mashed potatoes on the side.