1/2 medium head cauliflower
2 medium carrots
2 stalks celery
1 medium red bell pepper
2 cups white wine vinegar
1 cup water
1.5 tbsp kosher salt
1 tbsp granulated sugar
2 bay leaves
16 small pickled pepperoncinis
1/4 cup olive oil
2 tsp fennel seeds
1 tsp dried oregano
Cut 1/2 head cauliflower into bite-sized florets. Peel 2 carrots and thinly slice them crosswise on a slight diagonal. Thinly slice 2 stalks celery on a slight diagonal. Cut 1 red bell pepper into 1/4-inch-thick strips.
Bring 2 cups white wine vinegar, 1 cup water, 1 1/2 tablespoon kosher salt, 1 tablespoon granulated sugar, and 2 bay leaves to a boil in a medium saucepan over medium-high heat. Reduce the heat to maintain a simmer, add the cauliflower and carrots, and simmer for 3 minutes. Add the celery and red bell pepper and continue to simmer until all the vegetables are crisp-tender, about 2 minutes more. Remove from the heat.
Stir 16 small pickled pepperoncinis into the vegetable mixture. Cover the saucepan and let the vegetables marinate in the vinegar solution for 5 minutes. Meanwhile, prepare the marinade.
Place 1/4 cup olive oil, 2 teaspoons fennel seeds, and 1 teaspoon dried oregano in a large bowl. When the vegetables are done marinating in the vinegar solution, transfer 2 tablespoons of the vinegar solution to the bowl of olive oil. Whisk until combined and emulsified.
Pour the vegetables through a strainer or colander to drain, then add them to the marinade. Toss well to combine, then set aside until cool to room temperature, about 20 minutes. Cover with plastic wrap and refrigerate. Let marinate for at least 1 hour and up to 1 day before enjoying cold or at room temperature. Toss again before serving.
The giardiniera can be stored in an airtight container in the refrigerator for up to 1 week.