Add the coriander, mustard and cumin seeds to a medium saucepan. Toast the seeds over medium heat, swirling the pan around until the seeds are fragrant and slightly browned, about 2 minutes. Add the vinegar, garlic, ginger, onion, sugar, salt, peppercorns, turmeric, red pepper flakes and 1 cup water to the toasted spices. Bring to a boil.

Pack the cauliflower, carrots and bell pepper in a 2-quart jar or bowl. Pour the hot liquid over the vegetables. Let cool to room temperature, and then cover and refrigerate for at least 2 days before tasting. The veggies will keep for up to two weeks in the refrigerator.