Wash the orange and remove half of the peel with a zester in thin stripes. Press out juice. Mix orange juice and zests with horseradish, vinegar, maple syrup and olive oil in a large bowl. Season to taste with salt and pepper. Add prunes and rinsed mozzarella balls. Cover and let marinate for approx. 30 minutes. Wash and clean spinach leaves thoroughly with cold water. Let spinach leaves drain well or spin dry. Peel onion and cut in fine rings. Chop walnut kernels. Fold spinach leaves and onion rings gently in with the marinated prunes. Prepare on dishes and sprinkle with chopped walnuts.
Use arugula or fine mixed greens instead of spinach leaves.