One of the signature items of the California-based burger chain In-n-Out Burger is the "Animal Style" burger from their secret menu, which features a mustard-fried burger patty. It has a slightly tangy flavor to it, with a crispy, almost caramelized surface but a juicy center, and is often quoted as the best fast-food burger available.
I know the basic idea behind the process: squirt some mustard on the grill before putting the burger down. But I haven't tried it, and I'm curious if anyone has, and what specifically works best. Is it better to put the mustard on the burger, or directly on the grill? What type of mustard to use, and how much? At what point in the cooking process: just once before adding the burger, or a second time before flipping?
I know this falls under the restaurant-mimicry category, though I think I did a decent job of following the rules; If not, feel free to edit, or leave a comment and I'll try to clarify.