Ina garten's slow-roasted filet of beef with mustard-horseradish sauce

1. Preheat the oven to 275 degrees. Use an oven thermometer to be sure your oven temperature is accurate.

2. Place the filet on a sheet pan and pat it dry with paper towels. Brush the filet all over with the oil, reserving about half a tablespoon. Sprinkle it all over with the salt and pepper (it will seem like a lot but believe me, it makes a difference). Place the tarragon branches around the beef, tying them in 4 or 5 places with kitchen string to keep them in place, and then brush the tarragon with the reserved oil.

3. Roast the filet of beef for 1¼ to 1½ hours, until the temperature registers 125 degrees in the center for rare and 135 degrees for medium-rare. And about 2 hours for medium. I place the thermometer horizontally through the end of the beef. Cover the filet with aluminum foil and allow to rest for 20 minutes. Slice thickly and serve warm or at room temperature with Basil Parmesan Mayonnaise or Mustard-Horseradish Sauce.

4. Mustard-Horseradish Sauce:
Whisk all the ingredients together.uld be highly seasoned. Store in the refrigerator until ready to use; it will keep for up to a week.