1. In a bowl blend together the butter, shallot, tarragon and seasoning. Form the butter into a sausage shape and wrap in cling film or foil and refrigerate.
2. To make the pea puree place the onion, butter, stock and peas in a large pan, Season, bring to the boil, reduce the heat, cover and simmer for 4-5 minutes. Drain and reserve two tablespoons of the stock. Puree to a desired consistency with the reserved stock. Add the freshly chopped parsley, if used. Set aside and keep warm.
3. Gently heat 50g/2oz of the butter in a large pan until foaming. Add the fillet steaks and cook over a medium heat, turning once. Remove from the pan and keep warm.
For fillet steak 2-3cm (3/4 to 1 1/4inch) thick
Rare: 3-4 minutes (each side)
Medium: 4-5 minutes (each side)
Well done: 6-7 minutes (each side)
4. Add a dash of Port or cream sherry to the pan, stir, bring to the boil, reduce the heat and cook for a further 2-3 minutes until the sauce is reduced and syrupy.
5. Serve the steaks with the sauce on a bed of the pea puree with a hot new potato and chive salad.