These slightly crunchy pickled onions are a terrific contribution to a backyard barbecue or gathering as a relish for burgers, hotdogs, or sandwiches.
Place onions in a colander over a plate; sprinkle with canning salt and toss. Let stand 1 hour. Rinse and drain the onions, squeezing to remove excess liquid.
In a Dutch oven, combine vinegar, sugar and thyme; bring to a boil. Add onions and return to a boil. Reduce heat; simmer, uncovered, 10 minutes. Remove from heat.
Carefully ladle hot mixture into four hot half-pint jars, leaving 1/2-inch headspace. Remove air bubbles and adjust headspace, if necessary, by adding hot mixture. Wipe rims. Center lids on jars; screw on bands until fingertip tight.
Place the jars into canner with simmering water, ensuring that they are completely covered with water. Bring to a boil; process for 10 minutes. Remove jars and cool.