First, cut the greens into coarse pieces. Peel the onion and chop finely.
Heat butter and sauté onions and greens in it. Chop tomatoes with a kitchen knife. Add to the vegetables with the juice. Add wine. Season with salt, pepper, juniper berries and bay leaves. Simmer gently over low heat for 40 minutes.
Then pass through a sieve, bring to a boil again and season.
Pluck tarragon leaves and add to soup. Stir in gin to taste. Whip the cream (possibly with a dash of gin) until stiff.
Portion soup and serve with a dollop of whipped cream.