Peel and wash the potatoes. Cook in boiling salted water for 20 minutes. Then drain the potato water. Add milk. Mash finely. Peel onions and grate into the mashed potatoes.
Puree the caviar with the cutting stick of the hand mixer, fold into the puree. Season to taste with salt, lemon juice and olive oil. Serve cold garnished with rosemary and olives. Serve with fresh flat bread.