3–4 pounds dense, crisp vegetables peeled, if necessary and cut into bite-sized pieces. Such as, cauliflower, beets, carrots, radishes, fennel, asparagus, okra, or green beans.
Combine vinegar, water, sugar, salt, mustard seed, and chile flakes in a medium saucepan and bring to a boil over medium-high heat. Stir until sugar is dissolved, transfer to a large, heat-proof bowl and cool about 30 minutes.
Bring a large pot of water to a boil over high heat. Have ready a large bowl of ice water. Add one type of vegetables at a time to the boiling water and cook until crisp-tender. Transfer with a slotted spoon to the bowl of ice water. Once cool, spread out on a baking sheet and repeat with remaining vegetables, adding more ice to the water if necessary.
Add cooked vegetables to the pickling liquid and weigh down with a plate to keep submerged. Refrigerate, covered for 1 day before eating. Pickled vegetables will last 1 month, covered and chilled.