Boil the eggs: Place eggs in a large saucepan, cover with cold water, and bring to a boil over medium-high heat. Once boiling, reduce heat to low, cover, and simmer for 10-12 minutes. Transfer to an ice bath to cool, then peel carefully.
Prepare the brine: In a medium saucepan, combine vinegar, water, sugar, pickling salt, peppercorns, mustard seeds, garlic, bay leaves, and dill. Simmer over medium heat, stirring until sugar and salt dissolve. Remove from heat and cool slightly.
Pack the jar: Place peeled eggs and sliced onion (if using) in a sterilized 1-quart glass jar. Pour warm brine over eggs, ensuring they’re fully submerged. Add red pepper flakes for spice, if desired.
Seal and refrigerate: Seal jar with a tight-fitting lid and refrigerate for at least 3 days (1 week for best flavor). Shake gently every couple of days to distribute spices.
