Cook 10 trimmed and halved Brussels sprouts in a large stockpot of boiling water until bright green, about 5 minutes. Drain and rinse under cold running water. Bring 3/4 cup water, 1/4 cup white vinegar, and 1 tsp. coarse kosher salt to a simmer in stockpot over medium; simmer until salt dissolves. Combine 10 black peppercorns, 1/4 tsp. yellow mustard seeds, 1 sliced garlic clove, a pinch of crushed red pepper, and 1 fresh bay leaf in a sterilized pint jar. Place Brussels sprouts in jar; add hot vinegar mixture. (You may have some leftover liquid.) Cool to room temperature. Cover with lid, and chill 4 days or up to 2 weeks before serving. Store covered in refrigerator up to 1 month.