Place onions in a colander over a plate; sprinkle with canning salt and toss. Let stand 1 hour. Rinse and drain 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-in. 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 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.