Soak gelatine in plenty of cold water. Bring beef stock and 500 ml water to the boil. Season with vinegar and sugar. Let it cool down a little. Squeeze out the gelatine and dissolve in it.
Peel and wash the parsley leaves. Rinse a small box mould (approx. 1 litre capacity) with cold water. Add some stock, 1 slice of roast beef and parsley leaves. Leave to set in the fridge. Fill the tin with the remaining stock, roast beef and parsley leaves bit by bit. Let the aspic set in the fridge for 3-4 hours before turning it over (preferably overnight). For the sauce, peel the garlic and press it through the garlic press. Season mayonnaise with salt, pepper and garlic. Wash potatoes thoroughly and dab dry. Heat olive oil.
For the sauce, peel the garlic and press it through the garlic press. Season mayonnaise with salt, pepper and garlic. Wash potatoes thoroughly and dab dry. Heat olive oil. Fry the potatoes for 10-15 minutes, turning occasionally. Season to taste with salt, pepper and fresh thyme leaves. Serve aspic, potatoes and sauce garnished with fresh herbs