How to Make It
In a large, resealable plastic bag, combine the ale, carrot, celery and onion. Add the pork tenderloins, close the bag and refrigerate for at least 12 hours or overnight.
Remove the pork from the marinade and let return to room temperature, reserving the marinade. Preheat the oven to 375°. Transfer the marinade and vegetables to a saucepan and simmer over moderately low heat, stirring, until the vegetables are tender, 20 minutes. Transfer the marinade and vegetables to a blender and puree to form a smooth sauce. Return the sauce to the saucepan and season with salt and pepper. Keep warm.
Pat the pork dry with paper towels and season with salt and pepper. In a very large ovenproof skillet, heat the oil until shimmering. Add the pork and cook over moderately high heat, turning occasionally, until browned all over, 10 minutes. Transfer the skillet to the oven and roast the pork for 10 minutes, until an instant-read thermometer inserted into the thickest part of the meat registers 140°.
Transfer the pork to a cutting board and let rest for 10 minutes. Slice the pork crosswise and transfer to plates. Spoon the beer sauce over the pork and serve.