Add the cauliflower and cook until crisp-tender, about 2 minutes.
Drain and rinse under cold water until cool.
Pack the cauliflower, onion and garlic into a 1-quart jar.
Make the brine: Combine the red wine vinegar, distilled white vinegar, 3/4 cup water, the sugar, 1 1/2 teaspoons salt, the coriander seeds and lemon zest in a small saucepan and bring to a boil.
Pour the hot brine into the jar and let cool completely.