Preheat oven to 325˚F.
In a small mixing bowl, combine the oregano, thyme, rosemary, garlic powder, salt and pepper. Coat the beef tenderloin with the herb mixture.
Heat a heavy-bottomed skillet over high heat and add the olive oil once the skillet is hot. To the hot oil, add the tenderloin and sear each side for 3 to 4 minutes, or until golden brown.
Add butter, garlic, fresh rosemary, and fresh thyme to the skillet. Once the butter has melted, spoon it over the tenderloin.
Place the skillet in the oven, and roast for 30 minutes, or until the internal temperature reaches 135˚F to 140˚F.
Remove from oven; transfer to a cutting board and tent with foil. Let rest for 15 to 20 minutes before cutting.
While tenderloin is resting, make the horseradish sauce by whisking together the sour cream, mayonnaise, salt, and horseradish. Taste and adjust accordingly.
Slice the tenderloin and serve with the horseradish sauce.