Place a small skillet over medium-high heat until hot; add coriander and mustard seeds, and cook, stirring constantly, 2 minutes or until toasted. Set aside.
Fill a large bowl with ice water; place onion in bowl. Soak 20 minutes. Drain; pat dry. Return onion to bowl.
Bring red wine vinegar and next 4 ingredients to a boil in a small saucepan over medium-high heat. Pour over onion. Let stand 5 minutes. Drain. Stir in seeds and remaining ingredients. Store in an airtight jar in refrigerator until ready to use.
Small Bites, Big Nights (Clarkson Potter, 2007); Table 8, South Beach and New York