Preheat your oven to 375 degrees Fahrenheit.

Use a small melon baller to scoop out the top center portion of the onion about one-third of the way down to create a well. Place the onion on a sheet of aluminum foil and add the vegan butter into the well.

Mix the Cajun seasoning, salt, olive oil, and any optional seasonings in a small bowl. Pour this seasoning mixture over the butter into the well, allowing it to drip down the sides of the onion.

Wrap the entire onion tightly in aluminum foil, sealing it completely. Place the wrapped onion in a small baking dish or cast iron skillet.

Place the baking dish on the center rack of the oven and roast for 1 hour. Afterward, carefully open the foil and check for tenderness; the onion should be fork-tender. If more tenderness is desired, reseal and roast an additional 10 minutes before checking again.

For a golden, caramelized top layer, turn the oven to broil. Open the foil, drizzle additional melted vegan butter or olive oil on top, add a pinch more salt, and broil for 5-10 minutes, watching closely to prevent burning.

Peel away the outermost onion layer and serve each tender layer with a sprinkle of salt if desired. Pour all the flavorful juices collected in the foil over the onion.