This recipe for Govind Armstrong's pickled red onions is used in his recipe for Grilled Cheese with Pulled Short Ribs. The recipe comes from his cookbook, "Small Bites, Big Nights."
Toast the coriander seeds in a small, dry skillet over medium heat until fragrant, about 2 minutes. Add mustard seeds, and toast 1 minute more; set aside.
Soak onions in a large bowl of ice water, about 10 minutes. Drain, pat dry, and transfer to a medium heatproof bowl.
Place vinegars, sugar, bay leaves, and thyme in a small saucepan and slowly bring to a boil over medium heat; add reserved coriander and mustard seeds. If mixture seems too acidic, add a little water. Pour mixture over onions and let stand 5 minutes. Drain, discarding liquid, and season onions with salt and pepper.
Toss onions with olive oil and store, refrigerated, in an airtight container until ready to use, 2 to 3 weeks.