Set the mushroom on a cutting board with the bottom side down, as if it were a cauliflower. Use a long, serrated knife to cut from top to bottom into ¾-inch thick steaks, as evenly and uniformly as possible, which will help with even cooking.
Heat a cast iron or other heavy skillet to medium heat. Do not oil or add any liquid! Arrange the prepared steaks in a single layer and then set a heavy pot right on top of the mushroom steaks to press them down. You can also use a heavy brick wrapped in foil for this if you don't have a heavy pot. Let this cook for a couple of minutes without peeking!
Remove the heavy pot and carefully flip each steak with a spatula. You'll see a beautiful sear on the underside of the steak. Return the heavy pot on top of the steaks and cook another couple of minutes. Remove the pot and flip. You'll notice that most of the mushroom liquid has now evaporated.
Remove the seared steaks onto the preheated BBQ grill.
Return the searing skillet back to the stove, add 2 cups of reserved vegetable stock and plant-based Worcestershire sauce to the pan drippings, and bring to a boil. Stir with a whisk to bring up all the crispy, stuck-on mushroom bits. Let this reduce to about ⅓-½ cup and pour into a small bowl.
Return to the BBQ and brush the tops of each steak with this liquid, flip and grill, basting the other side. Repeat so both sides are nicely grilled and glazed.
Serve as desired. They are fantastic with mashed potatoes for a traditional "steak" and potatoes dinner. And save those little medallion pieces for your last bite, they are just amazing!