Preheat the oven to 400 degrees F.
Pat the tenderloin dry with paper towels and sprinkle all sides with a generous amount of salt and pepper – you should see the seasoning on the meat. Place a large skillet or roasting pan over medium-high flame, drizzle with the oil, and just when it begins to smoke lay the tenderloin in the hot pan. Brown on all sides until a crust forms and the meat is well-seared, about 7 minutes total. Toss the fresh thyme and rosemary on top of the tenderloin and transfer the whole thing to the oven; roast for 20 to 25 minutes for medium-rare.
Remove the tenderloin to a cutting board to rest for 10 minutes before carving. Pour off the excess fat from the pan and put it back on the stove over medium-high heat. Add the shallots to the pan drippings, saute, stirring with a spoon to scrape up the flavors in the bottom. Take the pan off the heat and pour in the cognac (premeasure - never pour directly from the bottle!) Put the pan back on the heat and tilt it slightly over the burner to ignite the alcohol, or light with a kitchen lighter. The cognac will flame for a few seconds then go out as the alcohol burns off. Stir in the demi-glace and cream, simmer for about 1 minute to thicken the sauce so it coats a spoon. Finish the sauce by stirring in the mustard and peppercorns until incorporated. Taste and season with salt, if necessary.
To serve: pile the watercress on a serving platter, cut the tenderloin into 1/2-inch thick slices and arrange it on top of the greens. Drizzle the sauce over the beef and garnish with chopped parsley. Serve the steak au poivre with pommes frites.
Recipe courtesy of Tyler Florence