Heat oven to 450 degrees
Loosely wrap the beets in tin foil and roast in the oven until they are fork tender, 30 - 45 minutes. Peel and dice the beets into small cubes.
Meanwhile, bring 3 cups of water to a boil. Add the barley and 1/2 teaspoon of salt. Reduce to a simmer, cover, and cook until the barley is al dente, about 30 minutes. If there is extra liquid in the pot, drain the barley and return it to the pan. Cover with a clean dishtowel until ready to use.
Combine barley, beets, red onion, feta, and scallions in a large mixing bowl. Sprinkle the lemon juice and olive oil over the top and toss to coat.
Taste the mixture. Add salt and pepper to taste.