Heat oil in saucepan. Add the onion, the garlic, the ginger, the fresh lemon grass (finely chopped or 1 tsp of ground lemon grass), the chili powder, and the turmeric to the oil. Cook on medium until fragrant.
Add water, fish sauce, small onions, and rice flour. Mix well, bring to a boil, and stir thoroughly to remove lumps. Once thickened, reduce to a simmer for ~20 minutes.
Cut fish into chunks, add to the soup, mix, and cook for another 10 minutes.
On the side, boil water and add rice noodles for ~5 minutes, until tender, and drain.