Preheat your oven to 400°F (200°C).

Peel the large sweet onions and slice them into thick rounds, about 1-inch thick.

Melt 4 tablespoons of unsalted butter in a small saucepan over medium heat. Add 3 minced garlic cloves and sauté for 1-2 minutes until fragrant.

Brush the garlic butter mixture generously onto each onion slice, covering both sides.

Arrange the onion slices on a greased baking sheet, drizzle with olive oil, and season with salt and pepper to taste.

Roast the onions for about 45 minutes, flipping halfway through for even cooking.

Remove from the oven, let cool slightly, and sprinkle with fresh thyme leaves before serving.