Thinly slice onion, and place in a 2-qt. glass bowl.
Heat a medium skillet over medium-high heat until hot; add cumin seeds, and cook, stirring constantly, 1 to 2 minutes or until toasted. Slowly stir in 1/2 cup water, lemon juice, and next 3 ingredients. Stir until brown sugar is melted and mixture comes to a boil.
Pour over red onion slices, and let stand 1 hour. Store in an airtight container in refrigerator up to 1 week.