Peel and thinly slice onion. In a 4- to 5-quart pan over high heat, bring 4 cups water, vinegar, mustard seeds, cumin seeds, and coriander seeds to a boil. Add onion, stir, and cook for 2 minutes. Pour into a fine strainer, discarding liquid. In a bowl, combine onion and spice seeds with lemon juice; salt to taste. Let cool at least 20 minutes, stirring occasionally.