Sophie Grigson's Agnello Al Limone e Finnochi

Ingredients

• A generous 2 kilos leg or shoulder of spring lamb on the bone, cut into large chunks, or 8 lamb leg steaks on the bone
• 30g (1 oz) butter
• 2 tablespoons extra virgin olive oil
• 1 large onion, sliced
• 120g (4 1/2 oz) prosciutto crudo or pancetta, cut into cubes
• 4 large fennel bulbs
• 200 ml (7 fl oz) dry white wine
• 5 large egg yolks
• 2 lemons
• Chopped parsley
• Salt and pepper