Heat oil over med-high heat until it shimmers, then add meat. Brown well on all sides, about 10 minutes. Sprinkle with pepper.
Turn heat to low and pour off most of the fat; save a couple tablespoons of the fat tho. Add garlic and chiles. Cook, stirring, for about 30 seconds. Add coconut milk and nampla. Raise heat; bring to a boil, then turn heat to low again. Partially cover pan. Adjust heat so that it simmers steadily.
Cook for at least one hour, stirring every 15 minutes. Do not let the bottom burn. If it becomes too thick, add water of about 1/2 cup or so at a time. Cook until meat is very tender.