Combine ingredients for spice sachet in a double layer of cheesecloth and tie tightly. In a large heavy bottomed pot, heat oil over medium high heat. Season pork with salt and pepper. Add pork to pot and sear on all sides, until very brown. Reduce heat to medium and add onions and garlic. Cook, stirring occasionally, until onions and garlic are tender and turning golden brown. Add tomato paste and spice sachet. Stir well to combine. Add chicken stock, reduce heat to low and simmer for 1 hour. Add brown sugar and fish sauce simmer for 15 minutes more.