Combine the sliced fennel, red onion, and garlic. Massage to release moisture.
Press for one or more hours to extract moisture.
Place in fermentation crock with liquid. Liquid should just cover the vegetables. If not enough liquid, add just enough water to cover slightly. Cover but don’t seal.
Allow to sit at room temperature (70 - 85°F.) until desired flavor is achieved, about 3 - 7 days. Move to refrigerator.