Add the oil and all seasonings to a large dish and mix. Brush it over the mushrooms, then leave the mushrooms in the remaining liquid to marinade.
Heat a griddle pan or clamp grill (like a George Foreman grill). Place the mushroom steaks onto the hot grill and cook until soft and lightly charred. If possible, save the mushroom cooking juices.
Alternatively heat a grill/grill pan/bbq/broiler and cook as above. Or, heat the oven to 200c/400f and bake on a tray for 25 minutes or until soft and juicy.
Serve with fries or mashed potatoes and steamed broccoli or other veg with the cooking juices poured over the mushrooms.
If you can't get hold of the liquid smoke (in larger grocery stores and online at Amazon), then these portobello steaks are still lovely without it. The nutritional information provided is approximate and is created with online calculators. Info will vary based on cooking methods and brands of ingredients used.