For simple yet delicious Spanish food there is a small bar-restaurant next to the "Chair of Felipe II", 2km away from "San Lorenzo del Escorial". For the last 30 years at least they have been making "tortilla de patatas" (potato omelet) and tomato salad. Both extremely simple but achieved to perfection. (you can also get chorizo, eggs, black pudding, but the speciality: Tortilla & Ensalada (omelet and salad).
---- some pics from jamama ----