Preheat oven to 450 degrees F.
Season tenderloin with salt and pepper on both sides. Place a large ovenproof skillet over high heat, melt the butter and sear tenderloin on all sides. Place in oven and cook to desired doneness or until an instant-read thermometer inserted registers 125 degrees F for medium rare, about 30 to 40 minutes depending on the thickness of the tenderloin. Let the meat rest for 10 minutes before slicing. Serve with the sauce.
Melt the butter and olive oil in a medium saucepan over medium high heat. Add the shallots and garlic and cook until soft. Add the rum and reduce to 1/4 cup. Add the stock, bring to a boil and reduce the heat. Whisk in the remaining ingredients and simmer until reduced to 2 cups.
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