Heat the oil in a saucepan over medium heat. Add the cumin, ginger, garlic powder, fennel and garam masala. Cook until the spices become fragrant, careful not to burn. Add the dried hibiscus flowers. Whisk together water, brown sugar, tamarind paste and soy sauce, add to the pan and bring the mixture to a simmer.
Continue to simmer the sauce for approximately 30 minutes, or until it’s thick enough to coat the back of a metal spoon. Strain the sauce through a fine sieve. Store refrigerated for up to five days.