1. Bring your tenderloins to room temperature for 1 hour prior to cooking. This will allow the tenderloin to cook evenly without a harsh grey band around the perimeter. 
2. Preheat oven to 400 degrees F with the lower-middle rack in position.
3. Generously season each tenderloin with flaky kosher salt, black pepper, and herbs de provence. 
4. Drizzle the avocado oil into a large oven-safe non-stick skillet over medium-high heat. When the oil begins to shimmer (you’ll see a webby appearance), sear each tenderloin for 1 to 2 minutes per side until a light golden brown crust forms. 
5. Immediately place the skillet with the tenderloins into your preheated oven, and let it finish roasting for 15-20 minutes or until an instant-read thermometer inserted into the center of the tenderloins registers 135 degrees. 
6. Remove the tenderloins from the oven and place them on a cutting board. Cover loosely with foil and allow the carry-over cooking to finish the tenderloin, about 10-15 minutes. This will bring the final internal temperature to 140-145 degrees F, which is where you want it. The center of the tenderloins should be a faint blush pink. 

For the glaze

1. Sautee the red onions in 1 tablespoon of butter in the same skillet over medium heat until softened, about 2-3 minutes. Season with salt, pepper, and crushed red pepper flakes. Add the garlic and rosemary, and cook, stirring for about 30 seconds. 
2. Carefully pour in the bourbon and allow it to dissolve about 70%, about 2 minutes. 
3. Add the cinnamon, soy sauce, brown sugar, honey, and remaining tablespoon of butter. Pour in the apple cider vinegar and allow the mixture to simmer and thicken for about 3 to 5 minutes. 
4. Taste for seasoning. When the flavor is on point, turn off the heat. Add in the tenderloins and generously baste them with the glaze.
5. Serve immediately with extra glaze on the side.