Preheat oven to 400°. Remove greens and stems from beet. Chop greens to measure 2 cups. Pierce beet and potatoes with a fork. Bake potatoes for 1 hour; bake beet for 1 hour 15 minutes. Peel beet and potatoes; press through a potato ricer into a bowl. Weigh or lightly spoon 4.5 ounces flour (1 cup) into a dry measuring cup. Combine potato mixture, 4.5 ounces flour, and 1/2 teaspoon kosher salt. Stir to form dough. Knead on a floured surface until smooth. Divide into 4 equal portions. Shape each into a 16-inch-long rope, dusting with remaining flour to prevent sticking. Cut each rope into 20 pieces. Roll each piece down the tines of a lightly floured fork; place on a baking sheet coated with cooking spray. Bring 6 quarts water to a boil in a stockpot. Add half of gnocchi; cook 2 1/2 minutes or until done. Remove gnocchi with a slotted spoon; place in a colander to drain. Repeat with remaining gnocchi. Heat a nonstick skillet over medium-high heat. Add onion; sauté 4 minutes. Add gnocchi, greens, 1/2 teaspoon pepper, and 1/4 teaspoon kosher salt; cook 2 minutes. Sprinkle with cheese.