Preheat the oven to 400°F and position the rack in the center.

In a large bowl, toss cubed butternut squash, chopped carrots, halved Brussels sprouts, and sliced red onion with olive oil, salt, and pepper.

Spread the seasoned vegetables in a single layer on a parchment-lined baking sheet, then roast 25–30 minutes until tender and golden.

In a medium bowl, whisk heavy cream, minced garlic, fresh thyme, salt, and pepper until smooth.

Layer the roasted vegetables in a greased 9×13-inch dish, then pour the creamy sauce evenly over them.

Sprinkle shredded sharp cheddar and panko breadcrumbs on top.

Lower the oven temperature to 375°F, then bake for 20 minutes until the cheese is melted and bubbly.

Allow the casserole to sit for 5 minutes before serving.