The depth of flavor from the Guinness marinade is enhanced by a touch of rich herb and garlic cheese that melts on the hot steak and onions. Freeze the cheese to make it easier to shave just enough on the top of the grilled steak and onions. This prevents too much cheese from being layered on when spreading the soft cheese.
Onions are easier to grill without falling apart if you skewer them before cooking. Cut an onion into 1/2-inch thick slices. Thread the center of each onion with a bamboo skewer that has been soaked in water. Place the onion skewers (they look like lollipops) on the cooking grates and turn once halfway through cooking.
Start to finish: 30 minutes active (plus 1 to 2 hours marinating)
2-pound flank steak or London Broil, at least 1-inch thick
14.9-ounce can Guinness beer
2 large red onions, cut into 1/2-inch slices
1 small container Boursin cheese
Ground black pepper
8 thick slices sourdough or country bread
Use paper towels to pat dry the steak, then set it in a glass or stainless steel container with a tight cover. Pour the Guinness over the steak and set aside. Cut the onion into 1/2-inch-thick slices. Set the slices on top of the steak, then cover the container and refrigerate for 1 to 2 hours.
Heat a gas or charcoal grill to medium. Set the cheese in the freezer.
Remove the meat and onions from the marinade and pat dry. Brush everything with a thin coat of olive oil, then season with salt and pepper.
Place steak and onions on the cooking grate over direct heat and sear for 6 to 8 minutes. Turn the steak and onions and sear on the second side for another 6 to 8 minutes.
(Alternatively, you also can sear the steak and onions on a stovetop grill or grill pan for several minutes per side, then finish cooking in a 350 F oven set on a rack over a sheet pan.)
Transfer the steak and onions to a platter. Remove the cheese from the freezer and use a vegetable peeler or paring knife to shave Boursin on top of the steak and onions. Cover with foil and set aside.
Brush each slice of bread with oil, then grill them until lightly toasted, about 1 to 2 minutes. Thinly slice the steak, then assemble sandwiches with the bread, steak and onions.
Nutrition information per serving (values are rounded to the nearest whole number): 1,250 calories; 380 calories from fat (30 percent of total calories); 42 g fat (18 g saturated; 0 g trans fats); 155 mg cholesterol; 121 g carbohydrate; 88 g protein; 6 g fiber; 1,870 mg sodium.