The secret at Doña Eutimia is that there is no secret: just serve decent-size portions of the best Cuban food. Expect the likes of ropa vieja (shredded beef; there's also an interesting lamb version), epic picadillo a la habanera (spicy beef), glorious lechón asado (roast pork) and beautifully rustic roast chicken, all served with ample rice, beans and fried plantains.
This is trip-defining food of the highest order, and proof that Cuba's traditional cuisine, when prepared properly, can be pretty spectacular. The restaurant is located in a cul-de-sac near the cathedral. Reserve a day ahead.