Heat olive oil in a large saucepan over medium heat. Add onions and garlic and cook until soft, about 4 to 5 minutes. Add ginger and continue cooking for 2 minutes. Add spices and cook for 2 to 3 minutes. Add tomatoes and their juice, water, brown sugar, tamarind paste and serrano and stir to combine. Cook for 30 to 40 minutes, or until thickened, stirring occasionally. Remove from heat, add cilantro and place in a food processor, process until smooth. Season with salt and pepper, to taste. Let cool.