Preheat oven to 400ºF. In a large heavy bottom pot, heat oil over medium heat. When oil begins to shimmer, add onions, 2 teaspoons salt, rosemary, and bay leaves. Cover pot, and cook over medium-high for 4-5 minutes, until they begin to look translucent and start releasing water.
Remove lid, and cook onions over low (or medium-low) until they caramelize. This will take 2-5 hours, depending on the size of your pot and the temperature used. The onions can be left alone and checked on about once an hour, except at the beginning and end of the caramelizing process.
While onions cook, peel most of the papery skin off the garlic. Cut about ¼ inch off the top of the garlic head (the side that comes to a point, not the side where the roots were). Set garlic heads on a sheet of aluminum foil, drizzle with a little olive oil, and fold foil over garlic. Place in oven for about 40-50 minutes. Garlic is ready when the center cloves are softened. (If the cloves feel soft, but are still pale, you can leave them in for another 10 minutes to allow them to caramelize more.)
Let garlic cool for a few minutes, and then push on the bottom of the garlic head to release the cloves. Set cloves aside.
When onions have reduced and are an amber color, you're ready. Add the garlic cloves, and mash them with the end of a wooden spoon. Add ginger, turmeric, and 5 cups of water. Bring to a boil, and then reduce to a simmer. Taste soup-- if you prefer a milder flavor, add another cup of hot water. Add additional salt to taste if desired. Remove the bay leaves.
Ladle soup into bowls, and serve immediately. Alternatively, store in an air-tight container in the refrigerator for up to 4 days, or in the freezer for up to 3 months.
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Notes
Cook Time:Soup takes 2 ½ - 5 hours to prepare, depending on how long the onions are caramelized. Most of the time is inactive, while the onions caramelize unattended.