1/4 cup olive oil
1/2 cup basalmic vinegar
2 TBSP Worcestershire
1 TBSP Dijon Mustard
4 gloves crushed garlic
2-3 pinches of fresh ground pepper
2-3 pinches kosher salt

The flap steak is a tougher cut of beef. Not sure if that would be mitigated by the Waygu quality or not. The vinegar and salt help to tenderize tougher cuts.

Use it on pork, beef, or chicken. Great with flank and pork tenderloin.

1/2 cup packed brown sugar
1/2 cup soy sauce
1/2 cup bourbon
1/4 cup Dijon mustard.

Whisk together, pour over meat in ziplock. Sit for a couple ours. Flip bag at least once.

It’s especially good on bone in pork chops:

1/4 cup bourbon
1/4 cup brown sugar
2 tbsp whole grain mustard
2 tsp vanilla extract.

Season the meat with salt and pepper and glaze 30-60 min before grilling, deglaze often while cooking.

I have to stay away from soy as my wife is allergic.

You don't need a marinade, but I almost always marinate it with a mixture of half soy sauce and half orange juice.

When you say "chunk," do you mean more like a flat strip? When I buy it, it's usually cut in strips about an inch or two wide and 6 or 8 inches long.

Get the grill as hot as you can and get a good sear on the outside, and the inside will still be medium rare and amazingly juicy. I'm getting hungry...

(Note: when I cook it, it's not wagyu. I don't know how much of a difference the extra marbling would make)

With a 1/4" fat layer on one side.

It'll drip and cause flareups on the grill.

Keep it simple and let the wagyu speak for itself.

But was second guessing. Plus, this approach will be a point of consistency with my normal process- allowing the cut to be compared as the main variable.