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 starting roasting it.
Preheat the oven to 225 degrees F. I like to take the fresh herbs and stick them right against the beef inside the kitchen twine. No rhyme or reason - just tuck them all over the beef!
Place the beef (still on the baking sheet or on a new one) 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 oven 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! Remove the beef from the oven and drizzle with more melted butter. Let the beef rest for another 10 minutes or so before slicing and serving with the caramelized onions and horseradish.
caramelized onions with thyme
Give yourself about an hour to do this! Heat a skillet over low heat and add the butter. Add the onions and salt and stir. Cook, stirring occasionally, until the onions caramelize and turn golden, about 45 to 60 minutes. They will be close to finished by then, but you can continue to caramelize for a few hours as long as you stir often. Keep an eye on them so they don't burn and keep the heat low! Stir in the thyme after 45 minutes.
Whisk all ingredients together (aside from about 1 tablespoon of chives for topping) and let sit in the fridge for 30 to 60 minutes. Top with fresh chives and serve!