Preheat the oven to 400 degrees F. Line a baking sheet with parchment paper.

Make the crispy cabbage. Place the sliced cabbage onto the parchment lined baking sheet. Pour the 2 teaspoons of olive oil over top, season with salt and pepper and add any spices you like. Toss lightly to combine and spread cabbage out evenly on the baking sheet. Slide the sheet into the oven and roast the cabbage for 13-17 minutes, or until browned on the edges and crispy in parts. You'll have to flip the cabbage around a few times while it's roasting. Keep it warm.

Make the chowder. Heat the olive oil in a large, heavy soup pot over medium heat. Add the diced onions to the pot and sauté until soft and translucent, about 5 minutes. Add the leeks and continue to sauté until the leeks are soft, about 5 minutes more. Add the minced garlic and thyme to the pot and cook until fragrant, about 1 minute, stirring constantly.

Add the chopped parsnips, celery root, and cauliflower florets to the pot and stir to coat in the oil. Add the grainy mustard, salt + pepper to taste, and Old Bay. Stir to coat all the vegetables in spice. Add the lmeon juice to the pot and stir. Add the vegetable stock to the pot, stir again, cover and bring to a boil. Once boiling, remove the lid and lower heat to simmer. Let the chowder cook and bubble until the parsnips and celery root pieces are tender, about 15-20 minutes.

Carefully ladle half of the chowder into an upright blender and carefully purée until smooth. Pour the puréed portion of chowder back into the soup pot and bring it back to a boil. Check the chowder for seasoning, adjust if necessary, and serve hot with some crispy cabbage on top