1. Trim the steak of all fat and cut into slices and then into 1/2 inch wide strips, cut these into 2 inch lengths. Peel and finely chop the shallots.
2. Melt half of the butter in a heavy frying pan (sauteuse) and when foaming, add half of the beef, turn strips over and seal quickly on all sides. They should remain rare inside.
3. Remove from the pan with a slotted spoon and seal the rest in the same way, remove from the pan and keep warm.
4. Add the remainder of the butter and when foaming, cook the shallots until melting but not colored.
5. Stir well, scrape up the meat juices from the bottom of the pan and add 4 tablespoons of cream.
6. Increase the heat and reduce the cream, stirring constantly until it thicken slightly and colors to the blonde stage.
7. Stir in the rest of the cream and the mustard, season well with salt and pepper.
8. Stir in the meat and juices it will have rendered, bring to a bubble but don't allow to boil.
9. If the cream sauce is too liquid, just before adding the meat, remove the pan from the heat and stir in the cornstarch mixed with a tsp of water.
10. Place the pan over low heat, bring back to a boiling point and allow to bubble for a minute before adding the meat and juices.
11. Do NOT boil again after adding them!
12. Serve immediately with rice or buttered noodles.