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Combine the sour cream, mustard and horseradish in a small bowl until smooth.
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Transfer into a refrigerator safe jar or container-this can be refrigerated up to two days ahead-and it really does make the flavors better.
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Grind the peppercorns (I use a cheap coffee grinder that I have designated for spices only) until coarse.
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Combine the coarsely ground peppercorns, salt, mustard, butter and parsley until well mixed.
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Rub the tenderloin with the spiced butter, coating completely-use all of the butter mixture.
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At this point you can wrap in plastic and refrigerate-up to 1 day-or go to the next step.
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Preheat oven to 450ºF.
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If you have refrigerated your beef, take it out and allow it to warm to room temperature.
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Place beef in a shallow baking pan (I use a small rectangular pyrex baking pan, you can also use a rack in your pan if you have one, but I have not found this necessary).
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Roast at 450ºF for 25-30 minutes, until your meat thermometer registers 130ºF (for medium rare).
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While tenderloin is roasting, thinly slice french bread and lay flat on a baking sheet or jelly roll pain.
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When tenderloin is done, take out of oven and allow to rest for 10-20 minutes.
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Slice beef thinly with a very sharp knife.
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Turn oven to broil.
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Place bread in oven on broil for 1-2 minutes, watching very closely to ensure bread does not start to burn.
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Allow bread to cool slightly.
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Spread horseradish sauce (made previously) on bread.
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Place arugula on top of sauce
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Place a thin slice of beef tenderloin on top of arugula
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If you are like me and love your sauces, you can place an additional dollop of sauce on the top.
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Serve and enjoy