Preheat the oven to 350°F.
With the cauliflower sitting on its stem, slice it into large slabs, about 1-inch thick. Slice it the same way you would slice a loaf of bread. You’ll get two or three large slabs.
Mix together the vegetable stock and chopped garlic in a 9x12-inch baking dish.
Place the cauliflower in the dish with the stock and cover tightly with foil. Bake for about 40 minutes, or until a knife can go into it easily. It should be cooked throughout but still a little firm.
Heat 1 teaspoon of oil in a large fry pan over medium-high heat. Gently place the cauliflower steaks in the pan. Let it cook for about 4 to 6 minutes on each side. You want to get it golden brown. Don’t fuss with it too much, or it will fall apart.
Set the cauliflower aside and keep warm.
Add the rest of the oil to the same pan and cook the onions for several minutes, until they become translucent start to brown slightly. Add the garlic, mushrooms, salt and thyme and continue cooking until the mushrooms have started to brown.
Add the Marsala wine and broth to the pan. Bring to a boil and then reduce the heat. Let simmer for 10 minutes or until the liquid has reduced by half.
To serve, place the mashed potatoes, cooked rice, or pasta at the bottom of a shallow bowl and top with the baby greens. Place a cauliflower steak on top of the greens, and then top with the Marsala mushroom mixture. Garnish with the chopped parsley. Serve hot.