Put the tongue in boiling water for 10 minutes, then pour off the water. Heat the tongue with the peppered onion, carrots, celery, leek, thyme and crushed peppercorns and 1 liter of water in a saucepan and cook for 2 ½ - 3 hours.
Remove the tongue from the brew, rinse with cold water and peel.
Cut 500 g of it into thin slices, use the rest for other purposes (e.g. serve cold).
Pass the brew.
Mix the egg yolks and cream. Mix the butter and flour into a roux. Deglaze with the white wine and stock. Cook over low heat to a creamy sauce.
After approx. 30 minutes add the egg yolks mixed with the cream. Bring to the boil, season to taste and stir in the capers. Cover the tongue slices with the sauce and serve.