Pre-heat the oven to 220°C.
Prepare the lime jus: In a heavy-based saucepan melt 20g unsalted butter, add the shallots and sweat for five minutes without colour. Add the tomato and garlic and cook for a further 3 minutes. Sprinkle over the sugar and stir until dissolved. Add the sherry vinegar and reduce until the vinegar and sugar have caramelised. Add the Port and Noilly Prat and reduce by three quarters. Pour in both stocks and bring to the boil, add the parsley stalk and reduce by a third. Pass the sauce through a fine chinois. Return the sauce to the pan and reduce by half. Whisk in the remaining 40g unsalted butter and season to taste with salt, freshly ground pepper and lime juice. Once more, pass through a fine chinois.
Reheat the roesti potatoes in the oven for 10 minutes.
Lightly season the calves liver with salt and freshly ground pepper and pass it through the flour. Shake off any excess flour from the liver. Heat the remaining 60g clarified butter in a large heavy-based pan and add the liver. Cook for 1½ minutes on both sides - the liver should remain pink inside. Over a low heat, warm the garlic confit, spring onions and batons of leek in the lime jus.
Place a roesti potato in the centre of a warm plate and set a slice of calves liver onto the roesti. Spoon the garlic, spring onion and baby leeks over the liver. Drizzle the lime jus over the liver and garnish with the lime segments and sprigs of chervil.