Heat the oil in a large Dutch oven or deep skillet over medium-high heat. Add the pork in a single layer and cook, stirring occasionally, until lightly browned on several sides, about 6 to 8 minutes. Work in batches if the pan is crowded.
Lower the heat to medium. Add the onion and cook until softened, about 3 minutes. Stir in the garlic and shrimp paste, then cook for 1 minute, stirring often, until fragrant.
Pour in the coconut milk, coconut cream, and water. Add the sliced chiles, brown sugar if using, and black pepper. Scrape up any browned bits from the bottom of the pan.
Bring the mixture to a gentle simmer. Reduce the heat to low, partially cover, and cook for 30 to 35 minutes, stirring every few minutes so the coconut sauce does not stick or split badly.
Uncover and simmer for 5 to 10 minutes longer, until the pork is tender and the sauce is thick enough to coat a spoon. If the sauce becomes too thick before the pork is tender, stir in a small splash of water.
Taste carefully. Add fish sauce only if the stew needs more saltiness; shrimp paste brands vary widely in salt level. Serve hot with steamed rice.