1. First, preheat your oven to 375°F (190°C).
2. Next, use a melon baller to scoop out the top center of the onion, creating a well about a third of the way down.
3. **After**, place the onion on a sheet of aluminum foil and add the vegan butter into the well.
4. Then, mix the Cajun seasoning with the olive oil and pour over the butter, allowing it to drip down the sides. Sprinkle with salt.
5. Next, wrap the onion tightly in the foil and place it in a baking dish to catch any drips.
6. Then, roast in the oven for 1 hour until the onion is fork-tender and slightly caramelized.
7. Finally, for extra caramelization, open the foil, drizzle with a bit more olive oil, and broil for 5-10 minutes until golden brown. Serve hot, peeling away the outer layer and enjoying each tender layer beneath. Don’t forget to pour any remaining juices from the foil over the onion for maximum flavor.