Mustard-fried burger patty 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.
Here is Kenji Alt's in-depth recreation of the double-double animal-style burger.  The core of his technique is:


  The process is simple: Sear the patty on one side, and squirt some
  mustard on it as it sizzles. Flip the patty over so that the mustard
  cooks into the second side.
  
  The patties are covered with the cheese, then the caramelized onions
  are applied liberally to a single patty before topping it with the
  second, fusing all the elements together into a single cheesy, beefy, sweet, oniony, gooey, salty, oozy, crispy, meaty, savory, melty, delicious mess.


He uses basic yellow mustard, btw.

In response to the question from comments about how long to cook, here is what the recipe says:


  Add burger patties and cook without moving until well browned and
  crusty on first side, about 2 1/2 minutes. While they are cooking,
  spread 1 tablespoon mustard on raw side of each patty with a spoon.
  Meanwhile, top each bottom bun with up half of spread, 4 slices
  pickles, 1 slice tomato, and lettuce. Flip patties with a thin spatula
  so mustard side is down and continue to cook for 1 minute