This Casco Viejo standby serves solid traditional Basque fare at bargain prices in an old-fashioned room with stone walls and exposed beams. In addition to standards like steak Wellington with foie gras, cod a pil pil, and oxtail braised in red wine, Aitxiar plays little tricks with some of the classics, like the grilled calamari with “crispy beards” served with potato and onion puree flavored with aioli. The roasted duck breast is browned on top and served with a port wine-chocolate sauce over yucca. The potato soup with cubes of fresh tuna is always good, as is the grilled foie gras over eggplant, zucchini, and grilled mushrooms.