Remove fat and sinew from fillets. Cut three quarters through lengthwise, open out, cover with wrapping film or greaseproof paper and flatten the fillets to about 5mm/¼in thick. Melt 55g/2oz butter in a saucepan and add onion, garlic and celery and cook gently until softened but not brown. Add remaining ingredients and bind with beaten egg and season with salt and pepper. Spread the stuffing on the two flattened fillets, roll them up and tie securely with string. Melt 25g/1oz butter and olive oil in the frying pan and brown the fillets. Butter a casserole with remaining butter, add the fillets and cook in a preheated oven at 200°C/400°F/Gas 6 for 40 minutes. Set the fillets aside and keep warm. Add the chicken stock and wine to the casserole and transfer the liquid to the saucepan, bring the liquid to the boil and then thicken with the beurre manié. Slice the pork fillets diagonally and moisten with a little of the sauce. Transfer the remainder of the sauce to the boat.