Preheat the oven to 275 degrees. In a large skillet, melt the butter in the oil. Season the pork with salt and pepper and brown over high heat, 3 to 4 minutes per side. Transfer to an enameled cast-iron casserole and add the garlic, onions, thyme, bay leaf, stock and 2 cups of water; bring to a boil. Cover with foil and bake for 2 1/2 hours, until the pork is falling-apart tender. Transfer to a dish; keep warm. Strain the liquid, return to the casserole and reduce by half over moderately high heat. Season with salt and pepper, add to the pork and serve with chimichurri.