Preheat the oven to 400˚ F. Put the beets in a large baking dish and add 1/3 cup water; cover the dish with foil. Roast, adding more water to the pan if needed, until the beets are easily pierced with a knife, 50 to 60 minutes. Uncover and let cool. Rub off the beet skins and cut the beets into 1/2-inch-thick wedges; transfer to a medium bowl.
Combine the orange juice, miso paste, sugar, soy sauce, ginger and garlic in a medium skillet over medium-high heat. Bring to a simmer and cook, stirring occasionally, until thickened and reduced to about 1/4 cup, 8 to 10 minutes. Strain through a fine-mesh sieve and pour over the beets. Season with salt and toss. Sprinkle with scallions.