In a medium saucepan, combine 1 cup of white vinegar, 1 cup of water, 1/4 cup of sugar, 1 tablespoon of salt, 1 teaspoon of black peppercorns, and 1 teaspoon of mustard seeds. Bring to a boil and simmer for 5 minutes until sugar dissolves.

Layer the ingredients in a clean glass jar: place 6 peeled hard-boiled eggs, followed by 1 pound of sliced sausage, and then add 1 thinly sliced onion.

Pour the hot brine over the layered ingredients in the jar, ensuring everything is submerged. Seal the jar tightly.

Let the jar cool on the countertop until room temperature, about 30 minutes. Transfer to the refrigerator for 24 to 48 hours before serving.