Preheat a grill to high on one side and lightly oil the grate. Season a 2-inch-thick porterhouse steak (3 pounds) with coarse sea salt and pepper. Grill over direct heat for 4 minutes, then rotate the meat 90 and cook 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, 12 to 15 minutes. Let rest 5 minutes before slicing. To make the sauce, simmer 3/4 cup balsamic vinegar, 2/3 cup ketchup, 1/4 cup honey, 2 sliced shallots, 2 tablespoons Worcestershire sauce, 1 tablespoon dijon mustard, 1/4 teaspoon allspice, and salt, pepper and sugar to taste until thickened, about 15 minutes. Strain, then whisk in some butter.