Trim Brussels sprouts and remove any loose outer leaves.
Bring a pot of salted water to a boil and blanch Brussels sprouts for 8–10 minutes until just tender.
Drain and let cool slightly.
Pack Brussels sprouts into a clean jar with garlic, mustard seeds, red pepper flakes, and peppercorns.
In a saucepan, heat vinegar, water, sugar, and salt until fully dissolved.
Pour hot brine over Brussels sprouts, fully submerging them.
Let cool to room temperature, then seal and refrigerate.
