Potatoes are cut into small cubes or planing fine grater. In boiling salted water cook the potatoes for 15 minutes. Add the oil and vegetable broth. Pour corn and peas. Boil for 10 minutes. Finally, add the spinach and simmer for about 5 minutes. Withdraw from heat to cool down. Thicken with the egg yolks and yogurt. Again put on the stove to boil gently while stirring.