Preheat your oven to 375°F (190°C).

Use a melon baller to scoop out the top center of the whole onion, creating a well.

Place the onion on aluminum foil, add butter to the well, drizzle olive oil, and sprinkle salt and Cajun seasoning on top.

Wrap the onion in aluminum foil tightly and place it in a baking dish.

Roast the onion for about 1 hour until it’s fork-tender.

Broil the onion for 5 to 10 minutes for added caramelization, watching closely.

Remove from oven, let cool, peel away the outer layers, and serve hot.