Preheat oven to 375°F. Cut stem off cabbage so it can sit flat. In a medium bowl, whisk together 3 tbsp olive oil, mustard, maple syrup, Worcestershire, and garlic powder, and season heavily with salt and pepper. In a large bowl, combine celery, carrots, onion, oil, and chopped herbs. Season with salt and pepper and toss to coat. Place vegetable mixture in a large, oven-safe skillet. Place cabbage in the center, on top of the vegetables, and brush all over with half the melted butter mixture. Pour half of vegetable broth into the bottom of the skillet and cover cabbage with aluminum foil. Bake for 45 minutes. Remove foil and brush with remaining olive oil mixture. Add remaining 1/4 cup broth and bake until cabbage is tender and slightly charred, 45 minutes more. (Pierce cabbage with a paring knife to check if it’s ready.) Meanwhile, make gravy: In a small saucepan over medium heat, melt vegan butter. Add onion and cook, stirring until soft, 6 minutes. Stir in mushrooms and herbs and season with salt and pepper. Cook, stirring occasionally, until mushrooms are soft and golden, about 4 minutes. Stir in flour and cook 1 minute, then whisk in 3 cups of broth and bring mixture to a boil. Reduce heat to low and simmer until mixture has thickened to your desired consistency, 5 minutes. (Add more broth if desired.) Slice cabbage into large wedges and serve with gravy. Garnish with parsley and serve hot.