In a saucepan over medium-high heat, warm the oil. Stir in the onion, lower the heat to medium, and cook, stirring often, until the onions have browned around the edges and softened, 6 minutes. Stir in the garlic and cook, stirring occasionally, until the garlic is very aromatic, about 1 minute. Stir in the shrimp paste and cook for 1 minute. Stir in the paprika, cayenne, turmeric, chile, and shrimp powder. Then stir in the tomatoes and bring to a simmer. Lower the heat to low and cook, stirring often, until the liquid from the tomatoes has mostly evaporated and the relish has thickened, 20 minutes longer. Add the fish sauce and salt and cook 1 minute longer. Season to taste with more fish sauce or salt.