1. Heat the oven to 425 degrees.
2. Mix the salt and sugar together in a small bowl.
3. Cut sixteen 8-inch-square pieces of aluminum foil and place two together so you have eightdouble-thicknesses of foil. Set the beets and onions on the squares of foil, one per square. Top each with 1 tablespoon butter and 1 tablespoon of the sugar/salt mixture. Sprinkle with mint leaves if desired.
4. Wrap each tightly and set on a baking sheet. Bake until tender, 45 to 60 minutes. Remove from the oven. Peel off the foil. Cut the beets into halves or quarters and the onions into bite-size pieces. Serve the beets with a mound of onions on the side. Squeeze with lime juice and sprinkle with coarse sea salt.