In a bowl, mix together the prunes, capers, olives, bay leaves, garlic, oregano, vinegar, wine, sugar, mustard, 2 tbsp olive oil and pour into a sealable container or plastic bag. Add the tenderloins and toss to coat evenly. Chill for a minimum of 2 hours to overnight.
Preheat to oven to 375°F. Remove pork from the marinade and pat dry, reserving the marinade. Heat 1 tbsp olive oil in a large ovenproof skillet over medium-high heat. Sear pork, turning occasionally until all sides are golden brown, about 3 minutes per side.
Allow the pan to cool for a few minutes before pouring in the remaining marinade. Place the pan in the oven to roast for 35-45 minutes, or until the pork is cooked through.
Transfer the pork to a cutting board allowing to rest for 10 minutes. Return the pan to the heat and reduce over medium-high heat, about 3-5 minutes. Swirl in cold butter.
Slice the pork crosswise against the grain and serve with sauce.
