Preheat your oven to 400°F (200°C). Pat the pork tenderloin dry with paper towels. Season generously all over with salt and pepper.
Heat the olive oil in a large oven-safe skillet over medium-high heat. Sear the pork tenderloin for 2-3 minutes per side, until nicely browned. Remove the pork from the skillet and set aside.
In the same skillet, add the minced shallot and cook for 1-2 minutes until softened. Add the minced garlic and cook for another 30 seconds until fragrant.
Add the chopped rhubarb to the skillet and stir to combine with the shallots and garlic. Cook for about 3-4 minutes, allowing the rhubarb to soften slightly.
Pour in the balsamic vinegar and chicken broth. Stir in the brown sugar and fresh thyme. Bring the mixture to a simmer, scraping up any browned bits from the bottom of the skillet.
Return the seared pork tenderloin to the skillet, nestling it into the rhubarb mixture. Spoon some of the sauce over the pork.
Transfer the skillet to the preheated oven. Roast for 15-20 minutes, or until the pork reaches an internal temperature of 145°F (63°C).
Remove the skillet from the oven. Transfer the pork tenderloin to a cutting board and let it rest for 5-10 minutes. Meanwhile, place the skillet back on the stovetop over medium heat. If desired, whisk in the tablespoon of butter to enrich the glaze. Simmer the sauce for another 2-3 minutes until it thickens slightly.
Slice the rested pork tenderloin and serve it over the cooked rhubarb mixture, drizzled generously with the balsamic glaze.