Boil the beets in 3 cups of salted water for 20 minutes or until fork-tender. Strain the beets and reserve the liquid. Let the beets cool and then cut them into chunks.
In the same pot, heat oil and saute red onion, celery, carrot and garlic until tender, about 6-8 minutes.
Add the beet cooking liquid, vegetable broth, and apple cider vinegar
Simmer for about 30 minutes. Add dill and kosher salt and black pepper to taste. For a smooth soup, strain the veggies out or keep them in for a chunkier soup.