Wash the leeks and cut into rings. Put 150 g aside. Peel the potatoes, onion and 2 cloves of garlic, chop coarsely and sauté with the remaining leek in 2 tbsp. oil for 5 minutes.
Deglaze with wine and allow to boil away. Pour in the soup and simmer for about 25 minutes. For the crostini, peel and chop a clove of garlic, mix with butter, salt and some thyme.
Spread on the bread slices and toast briefly under the oven grill. Puree the soup with the cream and season with salt and pepper. Sauté the remaining leek in the remaining oil and butter.
Season with salt and pepper. Top the soup with the leeks and serve with the crostinis.