Cut the fillet into slices about 3 cm thick. Heat oil and butter in a pan and fry the pieces of meat on both sides for 3-4 minutes over a medium heat, then remove from the pan and keep warm.
Peel the shallots, chop them finely and sauté in the frying fat for 1-2 minutes. Add the cream, vinegar and mustard. Reduce the sauce for 4-5 minutes, stirring constantly. Wash the herbs, pat dry, pluck the leaves and finely chop, stir into the sauce and season with salt and pepper. Arrange the fillets on a preheated plate and pour the mustard sauce over them.
Fresh baguette, but also potato croquettes or mashed potatoes, go well as a side dish.