Low-calorie potato, onion, and garlic gratin

Ingredients

 * 4 large baking potatoes, baked
 * ... scooped out and mashed
 * 2 large garlic heads, roasted
 * 2 large onions, baked
 * salt and pepper to taste
 * 2 tbsp buttermilk
 * 4 tbsp parmesan, grated
 * 1/4 cup skim milk

Directions
Remove the papery covering of the garlic heads, but do not peel and do not separate the cloves. Wrap well in foil and put them in the oven. Put the onions on a double sheet of foil, but do not wrap them; put them in the oven. Bake the garlic for one hour and the onions for an hour and a half at 42After an hour pull out the garlic heads, unwrap, and let cool for 5 minutes. Separate the cloves and squeeze them over a bowl so the softened garlic pops out into the bowl. After 1 ¼ hours, pull out the onions. If they are not very soft (almost collapsed) put them back in for a few minutes. With a sharp knife, cut off the stem and root ends of the onions. Remove the skin and the first layer. Put the onions in a food processor and puree. Reduce the oven to 350. Beat together the onions, garlic, and potatoes; and salt and pepper to taste. Beat in the buttermilk. Scrape the mixture into a gratin dish. Smooth the top and sprinkle with the cheese. (the recipe can be prepared to this point ahead of time and kept in the refrigerator for a couple of days. Bring to room temperature before proceeding) drizzle milk evenly over the top of the potatoes. Bake uncovered for 35 - 45 minutes, or until brown, bubbly, and thoroughly hot. serve at once