Don Martin is a charming, surprisingly inexpensive, restaurant with half the menu offering international food (with quite a few pastas) and the other half with more traditional dishes. It's located on a quiet street a block east and half a block north of the arch at 4a. Avenida Norte No. 27.
Dinner started with a lovely amuse of ceviche with homemade, super crispy, blue tortilla chips. For an appetizer I had the green mango salad from the traditional side of the menu and it was excellent. I had gone out specifically for a steak and there were three choices on the menu. I chose the one with a red wine and mushroom sauce. As are so many restaurant steaks here, this was a piece of filet mignon, not usually my favorite. But this one was very well prepared and was served with perfectly cooked miniature zucchini, squash, turnips, green beans, and carrots along with a scoop of slightly pasty rice that ended up being just fine for mopping up the mushroom sauce. The steaks, by the way, were the most expensive dishes on the menu and cost just a tad over $10.
There is a modest wine list, but a good selection among the limited offerings of half and quarter bottles. The co-owner/host/server speaks excellent English and is attentive without imposing. The restaurant is evidently quite well-known to locals (it's been around for nearly forty years; the chef and former manager bought the place from the original owner, Don Martin, about 8 years ago) but seems to have flown under the tourist radar. That should be remedied.