1 carrot, peeled
1 stalk celery
1 leek, white part only
1 medium-size white turnip, peeled
1 1/2 pounds fillet of beef, in four slices
Salt and black pepper
1 tablespoon extra virgin olive oil
3 cups beef stock
1 cup dry red wine
4 sprigs fresh thyme
1 teaspoon Dijon mustard
3 tablespoons soft butter
1 tablespoon minced chives
Cut carrot, celery, leek and turnip in 1/4-inch-thick matchsticks 2 inches long.
Season beef with salt and pepper. Heat oil in a 12-inch sauté pan, add beef and sear over high heat until browned but still almost raw inside. Remove. Add carrot, celery, leek and turnip to pan. Add stock and simmer over low heat until vegetables are tender, about 10 minutes. Remove vegetables to a saucepan, draining well to reserve stock, and cover.
Add wine and thyme to stock in sauté pan and reduce to about 1 cup over high heat. Remove thyme. Whisk in mustard. Over low heat, whisk in 2 tablespoons butter bit by bit to thicken sauce. Season with salt and pepper. Add beef and cook over low heat, basting with sauce until done, 4 to 5 minutes for medium-rare. Remove from heat and transfer beef and sauce to warm platter. Tent with foil.
Reheat vegetables in saucepan with remaining butter. Fold in chives. Arrange around beef on platter and serve.