Trim the sliver skin from the pork tenderloin. Slide a sharp boning knife, or other thin bladed knife, under the silver skin at one end. Once the cut is made, grasp the flap of silver skin and pull it taught while running the blade of the knife along the piece of meat under the silver skin. Angle the blade upwards at a slight angle to keep the slice thin. Keep pulling the flap of silver skin taut and the knife should easily slide under it. Discard the silver skin when done.
In a small bowl combine 1 1/2 tsp kosher salt and 1/4 tsp pepper with the garlic powder, onion powder, dried thyme, paprika, and poultry seasoning. Rub the seasoning into both tenderloins and let sit for at least 45 min or up to a few hours. Remove from the fridge a half hour before roasting to allow the meat to come up to room temperature.
In a wide skillet, add the butter and 1 tbsp olive oil and heat over medium heat until the butter begins to melt. Add all the onions along with a generous pinch of kosher salt. Cook for 3 to 4 minutes over medium heat until the onions begin to soften. Then turn down the heat to medium low.
Cook the onions over medium low heat until they are tender and golden brown. Keep an eye on the onions, stirring regularly, to ensure they caramelize evenly and do not stick to the pan. Add a bit more olive oil if the onions begin to stick. The onions will take about 45 minutes. Transfer the caramelized onions to a 9x13" baking dish and set aside.
Preheat the oven to 400°F. In a small bowl, combine the maple syrup, grainy mustard, minced garlic, balsamic vinegar, Worcestershire sauce, 1/2 tsp kosher salt and 1/4 tsp pepper. Whisk together and set aside.
Heat a skillet over high heat and add the remaining 1 tbsp olive oil. When the pan is hot, add the tenderloins and sear, turning occasionally, until the pork is golden brown on all sides. Transfer the tenderloins to the roasting pan with the onions.
Deglaze the skillet with the beef broth. Bring the broth to a simmer, using a wooden spoon to scrape up all the brown bits from the skillet. Add the maple syrup mixture and bring back to a simmer. Cook for 3 to 4 minutes until the sauce reduces slightly. Pour the sauce over the tenderloins.
Roast the pork for 15 to 20 minutes until it reaches an internal temperature of 145°F. Transfer the tenderloins to a cutting board and loosely tent with foil to rest. Allow the pork to rest for 10 to 15 minutes before carving.
To serve, slice the pork tenderloins into medallions. On a serving platter, arrange the caramelized onions from the roasting pan. Arrange the pork tenderloin slices on top of the onions. Spoon the remaining pan juices over the pork. This pork tenderloin recipes pairs perfectly with a parsnip and apple purée. Enjoy!