Preheat oven to 400°F (200°C). Lightly grease a 9x13 inch baking dish.

Bring a large pot of salted water to a boil. Add broccoli, cauliflower, and carrots. Cook for 8-10 minutes until fork-tender. Drain well.

In a large skillet, melt butter over medium heat. Add minced garlic and cook for about 20 seconds until fragrant.

Whisk in cornstarch and cook for 1 minute. Gradually whisk in heavy cream. Bring to a boil, whisking constantly, until the sauce thickens, about 3-4 minutes.

Remove skillet from heat. Stir in Italian seasoning, salt, pepper, chili powder, nutmeg, and shredded cheese until cheese is melted and sauce is smooth.

Spread 0.5 cup of the cheese sauce on the bottom of the prepared baking dish. Add the drained vegetables, season with salt and pepper, and stir to coat.

Pour the remaining cheese sauce over the vegetables and mix well. Sprinkle the top evenly with panko breadcrumbs.

Bake for 20-23 minutes, or until the casserole is hot and bubbly and the breadcrumbs are golden brown.

Let the casserole rest for 10 minutes before serving. Taste and adjust seasoning if needed.