In Madrid's Puerta del Sol/Plaza Mayor neighborhood alone, you can find some of its oldest restaurants offering traditional Spanish cuisine. There's Casa Botin -- the world's oldest -- and Casa Alberto, both operating since the 17th - 18th century. Joining their ranks is Lhardy which has been around since 1840.
From Puerta del Sol along Carrera de San Jeronimo, you can't miss Lhardy if only because one can't resist its window display of baked goodness like tocino del cielo, napolitanes, empanadas, ensaimadas, polvoron, marron glacé, etc. The ground floor serves very good coffee too while the upper floor is the fine-dining area.
I never had the chance to try their fine-dining restaurant and sample their cocido madrileño for which it is famous. Found it pricey especially if I'm dining alone. Luckily, at the time I visited their Tienda and ordered a cup of cafe cortado and a croissant, they were offering racion of Spanish staples like Callos Madrileño, Albondigas, and Tortilla. Priced at only 6€ in celebration of Lhardy's 175 years in the business!
Same goodness in a claypot but without the table linens and elegant cutlery in a private dining salon. Yet for all its aristocratic setting and fancy decor, the service was friendly and personal. The man behind the cash register tried to persuade me to take out some of the bakery stuff and I obliged, making him happy while putting my marron glacé in a beautiful Lhardy box. All but one, which I ate heartily in front of him.