1. Preheat oven to 400° F.
2. Heat a Dutch oven or heavy ovenproof skillet over medium-low heat and add 1 tablespoon of the olive oil. Add the onion, Italian seasoning, and ½ teaspoon salt and cook, stirring occasionally, until the onions are translucent and beginning to turn golden, 8-10 minutes. Stir in the water and remove from heat.
3. While the onions are cooking, prepare the cauliflower by removing the green leaves around the core, then cutting the stem flush so that the head sits flat.
4. Pour the remaining 1 tablespoon of olive oil over the cauliflower then sprinkle the remaining ¼ teaspoon salt over it. Clear the middle of the Dutch oven and place the cauliflower upright in it, and cover (if you do not have a lid, cover tightly with aluminum foil).
5. Bake the cauliflower 20-30 minutes, or until a paring knife meets no resistance when inserted. Increase the oven temperature to 450°F and remove the lid, basting the cauliflower with some of the juices. Continue to cook, basting occasionally, until browned, 20-30 minutes. Add ¼ cup water if needed to prevent the cauliflower from drying out.
6. While the cauliflower cooks mix the pesto and the vegan Parmesan (if using) together.
7. Remove the onions and cauliflower to a serving platter and drizzle the pesto over the cauliflower. Serve warm alongside extra pesto.