Wash the Napa cabbage thoroughly and finely shred it.
Peel the beets and grate them on a coarse grater. Slice the onions into thin strips.
In a small saucepan, mix vinegar, salt, sugar, oil, and pepper. Bring to a boil.
Combine all vegetables in a large bowl and pour the hot marinade over them. Let sit for one hour to absorb the flavors.
Transfer the mixture into jars, pressing it down to ensure the vegetables are covered with the liquid.