You probably want to move calling the initialization to constructor of the Menu class, providing that Menukort and Menu are same classes:
class Menu
{
public Menu()
{
CreateElementToMenu();
}
public List<Pizza> pizzaOnMenu = new List<Pizza>();
public void CreateElementToMenu()
{
pizzaOnMenu.Add(new Pizza(1, "Magarita", "Tomato, cheese", 49));
pizzaOnMenu.Add(new Pizza(2, "Hawaii", "Tomato, cheese, ham, pineapple", 55));
pizzaOnMenu.Add(new Pizza(3, "Cappriciossa", "Tomato, cheese, ham, mushroom", 55));
pizzaOnMenu.Add(new Pizza(4, "Special", "Tomato, cheese, beef", 60));
}
}