Roast beets and sweet potatoes together in same baking dish for about 45 minutes in 400 F or until you can pierce easily with fork.
If you are going to boil the Beets and Sweet Potatoes - Boil beets and sweet potatoes separately for about 25 minutes in high temperature until you can pierce them with a fork. Beets will stain the sweet potatoes if boiled together.
Once done, turn heat off, rinse the beets and sweet potatoes and let them cool for about 30 minutes.
Chop lettuce and arrange as desired in a deep salad bowl or flat salad platter (as the one in the picture).
Peel beets and cut in desired shapes, same with sweet potatoes. Place them over lettuce. Try to alternate to maximize the color effect.
Add chopped green onions and walnuts. Let the guests add salt, oil and balsamic vinegar to their serving or you can do it yourself and toss the the salad before serving. Either way it's perfectly fine:). Enjoy!!
Notes
You can roast or boil sweet potatoes and beets, your choice.