Drain beet juice from both jars into a pot. It should be about 2 cups.
Mix in sugar, salt, and apple cider vinegar and bring to a boil.
Turn down to a simmer and cook for 10 minutes.
Mix peeled eggs, sliced drained beets, and onions in a bowl or jar.
Pour hot juice mixture on top of eggs.
Seal in a jar or in a bowl with a lid. Place in the refrigerator for at least a day.