Is it an oxymoron to have to call 21 days in advance and hear a busy signal for 30 minutes to make a reservation at a casual Mexican eatery? If the food were sensational, I can understand, but what’s all the hype about? Well, the secret entrance doesn’t hurt. From the outside, La Esquina looks to be, and is, a three-dollar taco shop. But if you report to a woman with a clipboard, she grants you access to the swanky brasserie below.