The grains of rice disappear into the yogurt as it's simmered and stirred, thickening the soup and giving it substance. Serve warm or chilled, depending on the weather.
Whisk yogurt, rice, flour and egg in a large pot. Place over medium heat and cook, whisking constantly, for 1 minute. Add 3 cups water and bring to a simmer over high heat. Reduce heat to maintain a simmer and cook, whisking frequently to keep the rice from sticking to the bottom of the pot, until the rice is soft and the soup has thickened, 45 to 50 minutes.
Add herbs, watercress (or arugula) and salt. Cook, stirring occasionally, until the greens are softened, about 2 minutes. Stir in 1/4 cup water if the consistency is too thick. Serve warm or chilled, topped with cinnamon and more herbs, if desired.
1 starch, 1/2 whole milk