In a small skillet, melt the butter. Add the cumin and mustard seeds and cook over moderately high heat until fragrant, about 1 minute. Add the onion, garlic, chiles and ginger and season with salt and pepper. Cook over moderate heat, stirring occasionally, until the onion and garlic are browned, about 5 minutes. Remove from the heat and add 3 tablespoons of lemon juice, stirring to scrape up the browned bits from the bottom of the skillet.