Leave 1 tablespoon butter in a saucepan and sauté the finely diced onion in it. Squeeze the orange and stir the juice into the onion. Add the crayfish. Season with salt, pepper and tarragon.
Then stir the butter kneaded with approx. 1 teaspoon flour into the hot, non-boiling sauce until it is creamy. Round off with 2 tablespoons of whipped cream.
Serve as a starter with toasted bread corners or on tagliatelle as a main course.