Heat a small saucepan over medium. Add oil, then onion, garlic, chili powder, chili flakes and turmeric. Cook until onion starts to soften, 3 min. Add tomato, water and soy. Bring to a boil, then reduce heat to medium-low. simmer, uncovered, until tomato is soft and sauce is thickened, about 15 min. stir in vinegar and pepper. Purée mixture in a blender or with a hand blender. Serve with Tibetan momo dumplings.