Boil and peel the eggs. Place eggs in a pot, cover with an inch of water, and bring to a gentle boil.
Turn off the heat, cover, and let sit for 10–12 minutes. Transfer to an ice bath for 5–10 minutes, then peel. The ice bath makes peeling easier.
Prepare the jar. Wash the jar and lid in hot, soapy water and rinse well.
For best results, rinse with hot water right before filling to warm the glass slightly.
Make the brine. In a saucepan, combine white vinegar, water, apple cider vinegar, sugar, and salt. Add mustard seeds, peppercorns, red pepper flakes or chili, garlic, onion, and bay leaves. Bring to a boil, then simmer for 2–3 minutes to dissolve sugar and salt and release flavor.
Pack the jar. Place peeled eggs in the jar, layering with onion slices and garlic if you like.
Don’t smash the eggs; let them sit naturally so the brine can flow around them.
Pour the brine. Carefully pour the hot brine and spices over the eggs, making sure everything is submerged. If needed, top with a little extra vinegar and water (1:1) to cover completely.
Cool and chill. Let the jar cool at room temperature for about 30–45 minutes. Seal, then refrigerate.
Wait for flavor. For best taste, wait at least 3 days. One week is the sweet spot for classic pub flavor.
The eggs will continue to deepen in taste for up to 2–3 weeks.
Serve. Enjoy straight from the jar with a pinch of salt and pepper, a dash of hot sauce, or alongside pickles and chips. They’re great with beer, cider, or a sharp cheddar.