Start with the marinade:

Mix all ingredients with the meat in a medium bowl. Toss well. Cover and refrigerate overnight if possible, but just an hour or two is fine too. If you can't marinate, just move ahead with the recipe but salt and pepper your steak well before broiling.

Start by cooking your onions. Heat the oil in a medium skillet until hot--about 5 minutes. Add the onions and the sugar. Toss to coat. Sprinkle on a dash of salt and pepper and add the vinegar if you are using it. Cook on medium high until golden brown, stirring occasionally--about 15 minutes or so.

While the onions cook, preheat your broiler and insert a meat thermometer if using.

Broil the meat about 5 inches from the flame for about 5 minutes per side, or until the thermometer reads 145 degrees for medium.

Remove from the oven and allow to rest for five minutes before slicing.

Spread the buns with butter and toast in a skillet until just golden (or you can stick them under the broiler with the meat, but keep an eye on them).

Slice your meat into thin strips, no thicker than 1/4 of an inch wide across the grain.

Layer 1 tablespoon of mustard on the bottom of each bun. Add 1/2 cup arugula, followed by the steak and onions.