Cut off the beet leaves and set aside. Brush the beets thoroughly under running water and blot dry. Rub them with oil and a little salt and place on a baking sheet lined with parchment paper.
Bake in a preheated oven at 200ºC (approximately 400ºF / gas mark 3-4) and cook for 40-50 minutes.
Leave to cool slightly and then remove from their skins. Cut the flesh into pieces.
Peel the garlic and cut into slices. Rinse the lemon and cut into thin slices.
For the marinade, mix the vinegar with the olive oil and sugar and season with salt and pepper.
Mix the beets with the garlic, lemon and olives stir in the marinade. Leave to marinate for 30 minutes.
Rinse the beet leaves and remove some for the garnish. Cut the rest into strips. Mix into the beet salad and serve garnished with the leaves.