Popular in German households around the St Louis, MO area.
This is my version, adjust, and tweak for your tastes.
Use the type oil, vinegar, etc of your liking.
See footnote.

4 or 5 medium tomatoes
1/4 cup vinegar
1/4 cup sugar
2 TBS. oil
sliced sweet onions
salt & pepper to taste

Chop tomatoes, slice onions.
Whisk other ingredients together.
Mix, and refrigerate for a couple of hours or more, stir a few times before serving.

Footnote:
Many folks add sliced cukes and/or green peppers.
Other possible add-ins:
Green onions, hot peppers, feta cheese, cilantro, basil, and so on.