For the cippolini onions: Slice off the root end of the cippolinis. Put the cippolinis on a sheet pan and bake for 10 minutes in their skins. Remove from the oven, allow to come to room temperature, peel and reserve.
For the beef tenderloin: Tie the beef tenderloin and season it generously with salt and pepper. Heat a large skillet over medium-high heat. Add the olive oil, then the tenderloin. Sear in the skillet on all sides, about 2 minutes per side. Arrange the onions, garlic, thyme and carrots around the beef and add 2 tablespoons butter. Transfer to the oven and roast until the internal temperature registers 125 degrees F, about 15 minutes.
Remove the tenderloin from the skillet and let rest. Add 1 cup each of the beef stock and wine to the vegetables, along with the cippolini onions, and bring to a simmer, basting the vegetables occasionally. Cook until the liquid begins to reduce and thicken, about 15 minutes. Remove and reserve the carrots and cippolini onions and strain the liquid into a clean medium saucepan.
Add the remaining beef stock and red wine to the liquid in the saucepan and cook over medium-high heat until it reaches a glaze-like consistency, 10 to 15 minutes. Reduce the heat and, while whisking, add remaining 2 tablespoons of butter. Whisk until the butter is fully absorbed and the glaze has a nice sheen. Season with salt and pepper.
Cut tenderloin in half and submerge each piece in the red wine glaze, coating well. Remove beef, take off string and slice into 1-inch pieces. Arrange sliced beef on a platter and top with the reserved carrots, cippolini onions and watercress.