Heat oil in large soup pot. When hot, saute onion for 5 minutes, take care not to let it brown. Add potatoes and fry for, covered for 3 minutes. Add oregano, basil, parsley, salt & pepper. Cook covered for 2 minutes. Add tomatoes and cook for another 5 minutes. Add the stock, bring to a boil, reduce heat & simmer for 20 minutes, or until the potatoes are tender. Check seasonings. Let cool, blend till smooth. Reheat before serving, but take care not to let it boil.