Add the cooked beets, lemon juice, bay leaves, lemongrass, and ginger. Add water to cover the beets. Cover and cook at medium heat for about 10 minutes. Season with salt.
To make the soup, in a large pan melt the butter and saute the onions for 5-6 minutes.
Preheat oven to 150 degrees C (300 degrees F). Place the potatoes in a single layer on a baking sheet covered with parchment and slightly greased with oil. Bake for about 30 minutes then season and turn the potatoes. Bake for another 30-40 minutes or desired texture.