Guinness-Marinated Bison Steak Sandwiches

In a bowl, whisk the Guinness with the chopped onion, soy sauce, molasses, thyme, garlic, Worcestershire sauce and 2 teaspoons of pepper. Pour the marinade into a large, resealable plastic bag. Add the steaks and turn to coat. Seal and refrigerate overnight. Light a gas grill, then set it up for both direct and indirect grilling by turning off half of the burners. Remove the steaks from the marinade and pat dry. Brush with oil and season with salt and pepper. Grill over moderately high heat, turning once, until lightly charred, 6 minutes. Move the steaks to indirect heat, close the grill and cook until medium-rare, 8 to 10 minutes. Let rest on a carving board for 10 minutes. Slice 1/2 inch thick. Brush the onion rings and mushrooms with olive oil and season with salt and pepper. Grill over moderately high heat, turning occasionally, until softened, 6 minutes. Arrange the toasts on plates or a platter and top with the grilled onions, sliced steak and mushrooms. Drizzle with any juices from the bison steaks and serve.