Peel the onion and lard it with laurel and cloves. Boil 1 litre of water. Clean the mussels and wash them thoroughly. Discard opened mussels. Add mussels, onion, salt and peppercorns to the water and cook over medium heat for about 10 minutes.
Remove the mussels and let them cool down. Melt 30 g fat in a pot, dust flour over it and sweat it on. Gradually add stock, wine, milk and cream while stirring. Bring to the boil, season with salt, pepper, nutmeg and lemon.
Simmer for another 5 minutes while stirring. Peel and wash the carrots. Clean and wash the leek. Cut carrots and leek into fine cubes. Heat 1 tbsp. fat in a frying pan. Sauté the vegetables for about 3 minutes while turning.
Throw away unopened shells. Remove mussels from their shells. Wash thyme, shake dry and scrape leaves off the stalks, except for something to garnish. Add mussels and vegetables to the sauce.
Cut out the tops of the pies with a sharp knife. Heat in the preheated oven (electric cooker: 200 °C/ circulating air: 175 °C/ gas: level 3) for 2-3 minutes. Put the pâtés on plates and fill them with the ragout, place the lid on top.
Put the rest of the ragout on the plates. Garnish with thyme.