Heat clarified butter in saucepan. Lightly dust kidneys with seasoned flour and fry in hot butter. Turn over as they turn red on outside; set aside to drain.
Heat beef stock in saucepan and reduce by 1/2. Drain mushrooms and add mushroom liquid to beef stock.
In a separate pan, cook pepper and shallots in clarified butter on low heat. Add kidneys and stir, then add mushrooms. Stir in tomato paste, increase heat, add brandy and flame. Add white wine and reduced beef-mushroom stock. Mix together well and cook over medium heat. Add mustards and lemon juice and simmer 12 minutes. Lastly, add cream and increase heat again to heat through.