Place olive oil in a large sauce pan over medium heat. Add onions and cook until softened, about 10 minutes. Add greens gradually, a handful at a time. Stir until limp and slightly cooked, about 5 minutes. Stir in dill, then add 1 1/2 cups water and bring to a simmer. Turn off heat and let cool slightly. Place yogurt in a large bowl and stir smooth. While continuing to stir, slowly add the liquid from the pan. Return yogurt mixture to the plan and stir well. Cook over low heat just until warm. Do not boil or it will curdle. Add salt and pepper to taste. Ladle into warm bowls, sprinkle with sunflower seeds and serve at once.