Combine the vinegar, 3 tablespoons of sugar, chili, peppercorns, and garlic in a heavy, non-aluminum saucepan. Stir over moderate heat to dissolve sugar, then simmer for three minutes. Taste and adjust if needed with remaining sugar to balance sweet, tart, and spicy flavors. Add onion rings, then toss with chopsticks until onion wilts and liquid regains simmer, three to four minutes. Scrape mixture into a shallow bowl and refrigerate. Before serving, remove any loose onion membranes and most of the peppercorns.