Trim the silver skin and icky fatty parts from your flap meat, then cut into 3- or 4-ounce chunks, or whatever size you like. Place in a container with a tight-fitting lid; don't forget to leave extra space for the marinade.
In a large bowl, whisk together the soy sauce, Worcestershire sauce, molasses, brown sugar, red pepper flakes, black pepper and garlic. Add the stout; be careful, it will foam a bit. There is really no need to add salt--the soy and Worcestershire are plenty salty.
Pour the marinade over the beef tips and stir gently to evenly coat. (If you don't use all the marinade, it will keep, refrigerated, in an airtight container for up to 2 weeks.)
Refrigerate the meat for 8 hours or overnight; it will keep for up to 4 or 5 days. Cook as desired.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.
Recipe courtesy of Chef Wolf Lobster Shanty, Salem, MA