In a large saucepan, heat the olive oil. Season the pork all over with salt and pepper and add it to the pan. Cook over high heat, stirring occasionally, until browned all over, about 10 minutes. Add the onion and cook over moderate heat, stirring, until softened, about 4 minutes. Add the garlic and cook until fragrant. Add the wine and cook, scraping up any bits stuck to the pan, until reduced by half, about 5 minutes. Add the stock, capers, bay leaf, rosemary, thyme and tomato paste and bring to a boil. Simmer over moderately low heat, partially covered, until the meat is tender, about 30 minutes. Remove the lid and cook over moderately high heat until the sauce is thickened and reduced by half, about 5 minutes. Remove the herb sprigs and bay leaf and stir in the butter.