Peel shallots. Cut off the very tip and base, leave whole.
In a small saucepan, combine the shallots, sherry vinegar, red wine vinegar, sherry, sugar, salt, and pepper, and bring to a gentle simmer over medium heat.
Continue simmering for 60-75 minutes, until shallots are soft, and vinegar has reduced to a syrup.
Before serving, remove from heat and swirl in butter a few bits at a time to make a glaze.
Tenderloin Roast
Preheat oven to 375° F (190° C).
Allow roast to come to room temperature. Season well with salt and pepper.
Heat oil in a cast-iron skillet over medium-high heat.
Sear the roast evenly, one to two minutes per side, until well browned.
Reduce heat to medium-low and drain the oil out of the pan.
Add the butter, garlic, and thyme to the pan and start basting on one side for 3 minutes, then flip, and baste for 3 minutes more, until cooked through but still quite rare.
Remove from pan, transfer to oven and roast for 10 minutes to an internal temperature of 135° F (60° C).
Remove from oven, tent with foil, and let rest for 15 minutes before serving.
Serve
Slice the roast into four portions, present with the shallots.
Add a bit of the sherry reduction and the basting butter to the plate.
Garnish with fresh thyme leaves and flakey sea salt. Serve and enjoy!