When the original owners of Nielsen's first came to Houston from Denmark, they tried to sell the famous Danish open-faced sandwiches called smorrebrod. But customers didn't know how to eat the messy sandwiches (Danes use a fork and knife) and the health department wouldn't let them be displayed without some kind of covering. So Nielsen's gave up and started serving American-style overstuffed sandwiches with cole slaw, potato salad and deviled eggs on the side. Fifty years later, Nielsen's Danish deli on Richmond is a Houston landmark. But what made the place famous is a bit of a surprise: Houstonians come from far and wide to buy the homemade mayonnaise. While the thin-sliced pastrami on rye is excellent; and the corned beef, liver paste and Swiss sandwich is a work of art; it's the homemade mayonnaise-laden potato salad that caused Gourmet magazine to ask for a recipe. Nielsen's sells its mayo by the pint, too, so when you stop by for a sandwich, you can get some to stick in the fridge.