Mostaza offers an upscale, formal setting for an intimate dinner in the heart of Casco Viejo, yet it's tucked away in a discreet corner. Take advantage of the outdoor ambience and dine at an al fresco table in front of the historic ruin Arco Chato (The Flat Arch). Romantic. The Argentine-managed restaurant's specialties include beef cuts, seafood, and pastas. Do try the yucca croquettes.