Bright-hued sweet vegetable pickles are a welcome change from typical sandwich toppers. Bread-and-butters are tart, sweet, and slightly addictive.
Cut cucumbers and onion into 1/8-inch thick slices, and transfer to a colander set in a bowl. Toss well with the salt. Refrigerate 1 hour. Rinse well, and drain. Pat dry between paper towels. Transfer cucumbers and onion to a large bowl.
Bring vinegar, sugar, mustard seed, and celery seed to a boil in a medium saucepan, stirring. Reduce heat; simmer 4 minutes. Let cool slightly, 10 minutes.
Pour brine over vegetables; let cool completely, about 30 minutes. Transfer to airtight containers.