Place the beetroot tubers in the middle of a large piece of aluminum foil along with some garlic and oregano. Sprinkle with pepper and drizzle with oil and vinegar. Squeeze the aluminum foil and close it at the top. Cook on a baking sheet for 1-2 hours (depending on the size of the tubers).
Let beetroot cool before peeling. Then peel and cut into slices. Remove the garlic. Peel the oranges and divide them into segments. Layer the spinach, beetroot and orange alternately in a bowl. Make a dressing from the oil, vinegar, mustard and a little salt and pepper and pour over the salad. Serve immediately.