Wash all vegetables. Drain the pitted green olives and set them aside. Set out four clean pint jars.

Cut the cauliflower into small florets if needed. Slice the carrots, celery, and red bell pepper into small, bite-size pieces. Cut each serrano chile in half (lengthwise or crosswise, as you prefer); you will distribute ½–1 pepper per jar.

Place the following into each jar: 1 tablespoon olive oil, 1 smashed garlic clove, 1 bay leaf, ½ teaspoon dried oregano, ¼ teaspoon celery seeds, ¼ teaspoon fennel seeds, and ¼ teaspoon yellow mustard seeds.

Divide the cauliflower, sliced carrots, sliced celery, sliced red bell pepper, and the drained olives evenly between the four jars. Pack the vegetables in so the jars are filled to the top but still allow a small bit of space for the brine.

In a medium saucepan, combine 3 cups water, 3 cups white vinegar, 2 tablespoons salt, and 2 tablespoons sugar. Set the pot over high heat, bring the mixture to a full boil, and stir to dissolve the salt and sugar.

Carefully pour the boiling brine into each jar, fully covering the vegetables. If needed, press the vegetables down with a clean utensil so they are submerged. Leave a little headspace at the top of each jar.

Let the jars cool to room temperature (about 30–60 minutes). Wipe the jar rims if any brine spilled, then cover each jar with its lid and close tightly.

Shake each jar gently to distribute the oil and spices, then refrigerate. Let the giardiniera sit in the refrigerator for 36–48 hours before serving.