Rinse the beef tongue thoroughly under cold running water. Peel the onions and insert 2 cloves into each onion. Bring 1 liter of salted water to a boil in a large pot. Add the tongue, onions, soup greens, and bay leaf to the boiling water, adding more water if needed to just cover the tongue. Simmer over medium heat for approximately 2 hours, until the tip of the tongue is easily pierced with a fork. Remove the tongue from the pot and rinse under cold water. While the tongue is still hot, peel off and discard the skin. Trim away the gristly end. Return the tongue to the hot cooking broth.
For the Madeira sauce: Melt margarine in a saucepan over medium heat. Add flour and stir constantly until the mixture is medium brown, about 3-5 minutes. Gradually add 500 ml of the hot tongue cooking broth while whisking constantly to prevent lumps. Simmer over low heat for 5 minutes. Stir in Madeira wine, tomato paste, and cream, simmering gently without boiling. Season to taste with lemon juice, pepper, and salt.
Remove the tongue from the broth and cut into thick slices. Arrange the sliced tongue on a serving platter or plates and pour the Madeira sauce over it.