Roast or boil beets until tender (about 45 minutes for roasting at 400°F or 30-40 minutes for boiling). Cool completely and peel.
Slice 3 of the cooked beets and place in blender with 2 cups cold water or vegetable stock.
Add half the diced onion, garlic cloves, 2 sliced cucumbers, salt, pepper, vinegar, and most of the dill (reserve some for garnish).
Blend until completely smooth, about 2-3 minutes.
Taste and adjust seasoning with additional vinegar, salt, or pepper as needed.
Chill in refrigerator for at least 2 hours before serving.
Serve cold, garnished with reserved onion, dill, diced cucumber, remaining diced beet, avocado, olive oil, or yogurt as desired.