1. Combine 6 cups of water with the lentils in a 3- to 4-quart saucepan over high heat. Skim off the foam as the lentils begin to boil. Add tomatoes, oil, turmeric, and garlic. Reduce the heat to a simmer. Wrap ginger, rosemary, and bay leaf in a piece of cheesecloth, tie it closed with kitchen string, and add to the pan. Simmer until the lentils are tender and the garlic is soft; discard the cheesecloth.
2. Add the spinach and simmer until wilted. Stir in the lemon juice. Crush the garlic against the side of the pot with the back of a spoon and stir so that it melts into the soup. Season with salt and pepper to taste, and serve.