Chef Fran Rosat says stirring the rice is the quickest way to ruin a paella. Rosat runs the kitchen at Mare, a well-known restaurant in the Puerto Madero district of Buenos Aires. His argument is that a spoon knocks into the grains and frees their starch, which thickens everything into something closer to a risotto. A proper paella keeps each grain separate once the liquid has been absorbed. For that he calls for short-grain rice and a broth with real body, kept at a steady heat. The finishing touch is the socarrat, the toasted crust that forms on the bottom of the pan near the end. Getting it means leaving the pan alone and letting the rice sit until it crisps.