Bring a medium pot of water to a boil. 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. Cover and refrigerate overnight or up to 1 week.