Preheat the oven to 400°F. Spray a 9-inch square glass baking dish with olive oil spray. Cut the cabbages in half lengthwise. If you are using a large cabbage rather than 3 little ones, cut it in half, then cut each half into thirds so you’ll have 6 wedges. Using a paring knife, carefully create a small cavity in the cut side of each half, about the size of a grape, reserving the cabbage (you’ll have about ½cup) from creating the cavity.
Heat the olive oil in a large skillet over medium heat. Add the reserved ½cup of cabbage, the onion, and the soy beef. Using a fork, break up the soy beef to create little pieces. Sprinkle with sea salt and pepper. Cook for about 5 minutes, stirring occasionally until the onion and cabbage have softened and the ground beef is lightly browned. Stir in the cooked brown rice, mixing to combine. Taste for seasoning. Set aside.
Turn the tomatoes and their juice into a bowl. Using your hand, squeeze the tomatoes to crush them. Add the maple sugar, maple syrup, and vinegar. Sprinkle with sea salt and pepper. Stir to combine. Taste, and add alittle more maple syrup if you prefer a sweeter sauce.
Arrange the cut cabbage halves in a single layer, cut side up, in the prepared baking dish. Spoon the soy ground beef mixture evenly over the top. Spoon the tomato mixture evenly over and around the cabbage halves and over the beef mixture to cover. Place in the preheated oven and bake for 1 hour, or until the cabbage is tender when pierced with a fork and the sauce is hot and bubbling.