Preheat your oven to 350°F (175°C). While the oven is warming up, start by bringing a large pot of water to a boil. Carefully remove the core from a head of cabbage. Slowly add the whole head of cabbage to the boiling water. Allow it to cook for about 5-7 minutes until the outer leaves soften. Let it cool slightly, then peel off the leaves.

In a large skillet over medium heat, add olive oil. Once hot, sauté the chopped onion until it turns translucent. Toss in the minced garlic and cook for an additional minute. Next, add the shredded cabbage, grated carrots, celery, and spices. Stir everything together. If it looks too dry, add a splash of mushroom broth or water. Cook until the cabbage is tender and slightly caramelized.

In a large bowl, mix together the cooked rice blend, diced tomatoes, plant-based crumbles, and the sautéed cabbage mixture. Stir until everything is evenly combined.

Spread a layer of tomato purée on the bottom of a 9x13-inch baking dish. Now, carefully start filling the cabbage leaves with the rice mixture. Place each stuffed leaf seam-side down in the baking dish. If you run out of leaves, you can simply layer the filling on top.

In a separate bowl, combine the shredded cheese, cream, Dijon mustard, dried thyme, and egg. Once mixed, spread this cheese topping over the stuffed cabbage. It’s going to add a creamy, flavorful crust that you'll love.

Cover the baking dish with aluminum foil and bake for 30 minutes. Remove the foil and bake for an additional 15-20 minutes, until the cheese is golden and bubbly.

Once out of the oven, let the casserole sit for at least 5 minutes before serving. This will help it set and make it easier to portion out.