Heat 1 tbsp. butter and oil in a pan, add finely chopped onions, mushrooms and celery. Sautée them until there´s no jus into the pan. Season with salt, freshly ground black pepper, some marjoram, lemon juice and crushed garlic. Blend with herbs and allow the mixture to cool down. Cut pork fillet into slices (10 mm wide), cover with transparent film and pound well. Cover one half of fillets with 1 tsp. of stuffing, close fillets by pressing together the periphery and season with salt and freshly ground black pepper.
Heat some oil and roast fillets over lightly heat on both sides for some minutes. Remove fillets and keep warm. Pour beef stock into the pan and bring to the boil. Season the sauce for taste with salt and freshly ground black pepper. Refine with cream.