Ingredients

2 tablespoons coriander seeds 

1 tablespoon mustard seeds 

3 medium red onions, cut in half lengthwise and thinly sliced 

1/4 cup red-wine vinegar 

1/4 cup white balsamic vinegar 

1/8 cup sugar 

3 fresh bay leaves 

3 sprigs fresh thyme 

Coarse salt and freshly ground pepper 

1 tablespoon extra-virgin olive oil 

Preparation

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.