Preheat oven to 400°F. Toss Brussels sprouts with olive oil, salt, pepper, and garlic powder in a large bowl. Spread evenly on a baking sheet.
Roast the Brussels sprouts in the oven for about 25-30 minutes, flipping once halfway through, until they are browned and tender.
While the sprouts roast, melt butter in a saucepan over medium heat. Stir in flour and cook for 1 minute until golden. Gradually whisk in onion powder, Italian seasoning, milk, and cream.
Simmer for 3-4 minutes until the sauce thickens, then stir in 1/2 cup of the cheddar cheese and season with salt and pepper.