Wash one glass 1/2-gallon jar with a lid in hot, soapy water, and rinse well. Toast coriander in a small skillet over medium heat until fragrant, about 2 minutes. Add mustard seeds and toast for 30 seconds to 1 minute. Set aside.
Combine water, vinegar, salt, and sugar in a small saucepan, and bring to a boil. Cook until sugar and salt have dissolved. Add reserved coriander and mustard seeds; remove from heat, and set aside.
Fill jar halfway with either Vidalia or red onion slices. Add bay leaves and thyme; cover with remaining onion slices, packing them with the back of a clean spoon. Leave 3/4 inch of space beneath the rim. Pour hot liquid over onions covering them by 1/4-inch and leaving 1/2-inch of pace beneath the rim. Place lid on the jar, and let stand until cool. Store in the refrigerator, serve within a week.