Peel and roughly dice the onion. Peel, wash and roughly dice the potatoes. Add boiling water to the peas and drain them through a sieve after about 2 minutes.
Heat the oil in a pot. Briefly sauté the potatoes and onion and season with salt and pepper. Deglaze with broth and cook covered for about 20 minutes until soft. Puree the soup and let it cool down
Freeze 4 glasses (each containing approx. 300 ml). Wash the rosemary and remove the needles. Peel garlic and cut into thin slices. Put slices of bread on a baking tray and sprinkle with garlic and rosemary needles. Drizzle with olive oil and roast in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 5-10 minutes until golden brown.
Stir sour cream and whipped cream until smooth and season with salt and some sugar. Wash the mint, shake dry and pluck off the leaves, except for 2 stems. Add peas, buttermilk and mint to the cold soup and puree. Season soup with salt and pepper. Put the soup into the iced glasses. Pour cream on it and garnish with mint leaves. Add the crostini
Waiting time approx. 1 hour