The windowed wall at M Burger, a month-old takeout spot attached to Tru, offers a peek into the restaurant's bustling kitchen. But that's not the only sneaky thing about the place.

In the grand tradition of In-N-Out, M Burger has a secret menu. It's short and sweet, but like all secret menus, it's home to a few specialties that shouldn't be missed.

The classic M Burger, with a griddled patty (or two), caramelized bacon and mayonnaise-based sauce ($3.30), is a fine to-go burger specimen. But sauce and spice fiends should request the off-menu Hurt Burger, which is slathered in a sweet and spicy barbecue sauce and topped with pepper jack cheese ($3.50 for a single; $5 for a double).

Order the Hurt and request your level of pain: The first degree spreads barbecue sauce on one bun and mayo on the other; the second degree adds a layer of sliced jalapeños; the third is extra-saucy, crowned with a mound of the fresh green chiles.

The on-menu Nurse Betty ($3) is a vegetarian's friend, with a thick slice of beefsteak tomato sandwiched by guacamole, mayonnaise, lettuce and pickles. Request an off-menu Dr. Betty ($4), and a beef patty will make its way into the mix; think of it as a burger and a salad rolled into one.

Craving simplicity? You have the right to request a cheddar-on-white-bread grilled cheese ($2). Even sweeter, you have the option to finish with an off-menu butterscotch milkshake ($3), a caramel-y, creamy dream served in a to-go cup.

M Burger, 161 E. Huron St.; 312-254-8500 or mburgerchicago.com