Place the eggs in a large pot and cover them with cold water. Bring to a boil, then remove from heat, cover, and let sit.

Drain the eggs and rinse under cold water, then let them cool just enough to handle. Peel and place them in a clean glass jar.

In a saucepan, combine vinegar, water, most of the onion slices, salt, pickling spice, and peppercorns. Bring to a rolling boil.

Carefully pour the hot brine over the eggs in the jar. Top with the remaining onion slices.

Seal the jar and let it cool to room temperature.

Refrigerate for at least three days before serving so the flavors fully develop.