Crust is a term applied to a puff pastry or brisée wrapper in which chicken, fish, meat or vegetables are enclosed and mixed with other ingredients, such as eggs, cheese etc. One of the most typical examples of crust preparation is the Wellington fillet: a piece of beef tenderloin cooked briefly over high heat, then left to cool, spread with mustard and mushroom purée (or truffles), wrapped in slices of cooked ham and a puff pastry wrapper, finally cooked in the oven. Other crust preparations, made in special molds, require the pasta to perfectly enclose the food, so that the ingredients (especially if small in size) can not come out of the casing during cooking. Furthermore, to avoid an excessive formation of steam inside the "crust" , which could cause it to swell or even split, a hole is made at the center of the dough and a cardboard cylinder is inserted into it, acting as a breather (this is in fact called chimney). The crust must be shiny and golden and for this reason it must be brushed with some yolk.