- 4 cups 1/2″ cauliflower florets (cut from 1 large head)
- Kosher salt
- 1 cup apple cider vinegar
- 2/3 cup finely chopped onion
- 5 1/2 tablespoons sugar
- 4 teaspoons yellow mustard seeds
- 1 1/4 teaspoons (generous) dry mustard
- 1 1/4 teaspoons (generous) celery seeds
- Cook cauliflower florets in a large pot of boiling salted water until just crisp-tender, about 2 minutes. Drain cauliflower. Transfer to a rimmed baking sheet and set aside.
- Meanwhile, combine vinegar, onion, sugar, mustard seeds, dry mustard, and celery seeds in a large saucepan. Stir over medium heat until sugar dissolves.
- Add cauliflower to saucepan; bring to a boil. Reduce heat to medium and simmer until cauliflower is tender, about 3 minutes. Transfer mixture and pickling juices to a 1-quart jar. Let cool slightly, cover, and chill. Serve within 1 month.