Pickled Vegetables

For pickling liquid
2 1/2 cups distilled white vinegar
3 cups water
3/4 cup sugar
5 tablespoons kosher salt
1 teaspoon yellow mustard seeds
1/2 teaspoon dried hot red-pepper flakes

For vegetables
1 head cauliflower (2 lb), trimmed and broken into 1- to 1 1/2-inch florets (6 cups)
1 red bell pepper, cut into 1-inch pieces
1 yellow bell pepper, cut into 1-inch pieces
4 carrots, cut diagonally into 1/2-inch-thick slices (2 cups)
4 celery ribs, cut into 1-inch-thick slices (3 cups)
1 cup drained bottled whole peperoncini (4 oz)
1 cup large brine-cured green olives (preferably Sicilian; 6 oz)
1/2 cup oil-cured black olives (6 oz)
Preparation
Make pickling liquid:
Bring pickling-liquid ingredients to a boil in a 3-quart nonreactive saucepan over moderate heat, stirring until sugar is dissolved. Transfer to a 4-quart nonreactive bowl and cool about 30 minutes.

Cook vegetables:
Bring about 6 quarts unsalted water to a boil in an 8-quart pot. Have ready a large bowl of ice and cold water. Add cauliflower to pot and boil until crisp-tender, about 4 minutes, then transfer with a slotted spoon to ice bath to stop cooking. Cook remaining vegetables separately in same manner, allowing 4 minutes each for bell peppers and carrots and 2 minutes for celery. Drain vegetables in a colander and spread out on 2 large kitchen towels to dry.

Add cooked vegetables, peperoncini, and olives to pickling liquid. Weight vegetables with a plate to keep them submerged, then chill, covered, at least 1 day.