Preheat the oven to 375°F (190°C).

Place the trimmed onions in a ceramic baking dish and brush surfaces with melted butter.

In a medium bowl, whisk together heavy cream, minced garlic, thyme, salt, and black pepper until well combined.

Pour the cream mixture evenly over and around the onions in the baking dish.

Cover the dish tightly with aluminum foil and bake for 30 minutes to soften the onion cores.

Remove the foil and sprinkle the grated Asiago cheese and panko breadcrumbs over the tops of the onions.

Return to the oven and bake uncovered for an additional 20 minutes until the onions are tender and the cheese topping is golden brown and bubbly.

Remove from oven and let rest for 5 minutes to allow the cream sauce to thicken before serving.