- 2 medium zucchini (about 12 oz.)
- 2 medium-size yellow squash (about 12 oz.)
- 1 red onion, halved and cut into 1/8-inch-thick slices
- Parchment paper
- 2 1/2 tablespoons kosher salt
- 1/2 cup apple cider vinegar
- 1/2 cup rice vinegar
- 3/4 cup sugar
- 1 teaspoon celery seeds
- 1 teaspoon mustard seeds
- 1/4 teaspoon dry mustard
How to Make It
Cut zucchini and yellow squash lengthwise into 1/8-inch-thick slices, using a mandoline or sharp knife. Spread zucchini, yellow squash, and onion in a single layer on 2 parchment paper-lined baking sheets. Sprinkle with salt. Let stand 1 hour. Transfer to a colander, rinse, and drain. Place in a widemouthed 1-qt. jar, filling to 1/2 inch from top.
Bring vinegars to a boil in a medium saucepan over medium heat. Add sugar; cook, stirring constantly, 3 to 5 minutes or until dissolved. Stir in next 3 ingredients. Bring to a boil, immediately remove from heat, and pour over vegetables. Cool 1 hour. Cover and chill 3 days before serving. Refrigerate up to 2 months.