Peel 4 medium beets and cut into 1/2-inch pieces. Toss with 1 tablespoon olive oil, 1 teaspoon salt, and pepper to taste on a baking sheet. Roast at 450 degrees F, stirring once or twice, until tender, 35 minutes. Transfer to a bowl; toss with 4 chopped scallions and 2 teaspoons lemon juice. Top with crumbled feta.