Steps
1. In a large pot, combine the potatoes, the water, and the salt and bring to a boil. Reduce the heat, cover, and simmer until the potatoes are cooked for about 10-15 mn. Drain and reserve the cooking liquid.
2. Meanwhile, melt the butter or margarine over medium heat in a saucepan. Add the grated onion and the cornstarch, and stir until blended.
3. In another bowl, bring the milk to a boil and add at once to the cornstarch-butter mixture saucepan, stirring constantly. The mixture will thicken when it comes to a boil. Season with salt (optional) and nutmeg.
4. Puree the potatoes in an electric blender until creamy, using the reserved cooking liquid for this purpose. Add the cooked beets and blend until smooth.
5. Reheat and add the thickened milk mixture, stirring until thoroughly mixed.
6. Serve hot sprinkled with croutons and Parmesan cheese.

Source
Dominican Cookbook

Serving
6 - 8