The key to a delicious beef tenderloin is to start a day or two before! Ideally you want to tie the filet up with kitchen twine so it cooks evenly. You can do this with the method of using butcher's knots - there is a great tutorial right here.
Place the tenderloin on a wire rack that's sitting on a baking sheet. Season the meat liberally with the salt and the pepper. Place the sheet in the fridge and let the tenderloin sit uncovered overnight.
Be sure to pull the beef from the fridge at least an hour or so BEFORE you want to start roasting it.
Preheat the oven to 225 degrees F. Place the tenderloin in a roasting pan with the onions around it. Stir together the sugar, espresso powder and garlic powder. Rub it all over the tenderloin.
Place the roasting pan on the center rack in the oven and roast for 2 1/2 to 3 hours (for a 3-pound tenderloin) or until the internal temperature reaches 120 to 125 degrees F on your meat thermometer. Remove the beef and set it aside for 10 minutes.
During those 10 minutes, place the butter in a saucepan and let it melt. If it verrrry slightly begins to brown, I let it, but I don't let it brown too much because it's going to be under the broiler!
Adjust the oven rack so it's about 6 inches from the broiler and heat the broiler to high. Spoon some of the melted butter over the roast and place it under the broiler. You want to turn the beef every 30 seconds (and I also spoon more butter on it!) until the outsides are golden and browned. You only want to do this for about 2 to 3 minutes total - keep an eye on it!! Remove the beef from the oven.
Lift the beef from the roasting pan and place it on a cutting board to rest. Place the roasting pan over a burner and heat to medium-low heat. Stir any bits from the bottom of the pan and let the mixture simmer for a minute or two, so it thickens. You can use this as a drizzling sauce for the filet!
Slice the filet into 1-inch slices and serve with the truffle risotto. Note: I added the rosemary on top for garnish for these photos! But you can do it to help it look pretty on the table.