Rob and Maggie Wasserman, whose burgers at Rouge on Rittenhouse Square get national attention, are less than two weeks from opening a burger-only shop called 500º at 1504 Sansom St., about four blocks away.

Naturally, much R&D went into the 500º burger. Rob Wasserman explains that they don't want to replicate the Rouge burger, a 12-ouncer topped with Gruyère and caramelized onions and served on a challah roll.

At 500º, chef Matt Zagorski is going with 5½ ounces of beef, also on a challah roll from Wild Flour Bakery, which will deliver twice a day.

With the 500º opening set for March 23*, a dozen loyalists and insiders sat at Rouge Wednesday morning to give feedback and taste-test the menu, which is simplicity itself: cooked-to-order burgers (with only four toppings and four cheeses), fries, shakes, and soft drinks.

In sum, the Wassermans are aiming to serve a delicious sub-$6 burger, the culinary equivalent of the 4-minute mile. Ergo, a meal for $10, or $12.50 if you get a shake.

For this odd breakfast, Zagorski put out variations of the plump burgers, topped with bacon, mushrooms, jalapenos, and grilled red onions -- cheese choices are American, provolone, Swiss, and Cheddar. The juices sank deeply into the buttered challah roll. Not too messy, and definitely not a flat, fast-food-type patty.

The signature burger, the 500, comes with bibb lettuce and tomato (just as the Rouge burger does), along with Cheddar, bacon and a hauntingly delicious "special sauce" that I know contains Worcestershire sauce and mayo. Customers can request the doneness; Wasserman likes his burgers medium rare, and a few of the test burgers came out practically mooing.

The eatery -- which will have limited seating -- will offer three kinds of shoestring-cut fries: plain, spicy (which really are), and truffle. Zagorski shook truffle salt on them to achieve the flavor. There's a spicy aoili dipping sauce for the fries.

He sent out small glasses of shakes (chocolate, vanilla, black-and-white), made of local Bassett's ice cream because that's what Wasserman used as a kid.

Packaging will be recycled. There will be no trays. Each item will be bagged.

(Major props to the Wassermans. Back in October, he said the eatery would open in the first quarter of 2010.)

* As for opening day, 11 a.m. March 23, here's the deal:

The first 100 people will get any burger for free.

After those 100 people, they will run a new deal until 3 p.m.: If you bring a friend and order a burger, the friend gets any burger of equal or lesser value free.

From 5 to 7 p.m., you get free soda and fries with any burger.

From 11 p.m. to close, you get free fries with any purchase.

(See the accompanying quickie video, an experiment with a Flip camcorder.)

Menu here.