Preheat a grill to high on one side and lightly oil the grate.
Season the porterhouse steak with coarse sea salt and pepper.
Grill over direct heat for 4 minutes, then rotate the meat 90 degrees and cook for 2 more minutes.
Repeat on the other side.
Move the steak to the cooler part of the grill (position the smaller end away from the heat); cover and cook until the meat reaches 128 to 130 for medium-rare, about 12 to 15 minutes.
Let rest for 5 minutes before slicing.
To make the sauce, simmer balsamic vinegar, ketchup, honey, sliced shallots, Worcestershire sauce, dijon mustard, allspice, and salt, pepper and sugar to taste until thickened, about 15 minutes.
Strain the sauce, then whisk in some butter.
