Roast the vegetables.

Preheat the oven to 425 degrees.

Line two rimmed baking sheets with aluminum foil and set aside.

In a large bowl, toss the broccoli florets, olive oil, minced garlic, salt, and pepper together until the broccoli is well coated.

Spread the vegetable out in a single layer on one of the baking sheets.

Repeat the same process with the cauliflower florets, mixing them with the oil, garlic, salt, and pepper until coated.

Spread the cauliflower out on the second baking sheet.

Bake both the broccoli and cauliflower in the preheated oven for 10-12 minutes until they are tender-crisp.

Remove the trays from the oven and set them aside.

Meanwhile, make the alfredo sauce.

In a large saucepan over low heat, melt the butter and cream together.

Simmer the mixture for two minutes.

Whisk in the garlic, Italian seasoning, salt, and pepper.

Cook, whisking constantly, for another minute.

Add the cheese and nutmeg.

Whisk the mixture until the cheese has melted and the sauce has thickened slightly.

Remove the pan from the heat.

Assemble the casserole.

Reduce the oven temperature to 375 degrees.

Grease a 2 ½-quart casserole dish with cooking spray.

In a large bowl, add all of the cauliflower, and most of the broccoli florets. Reserve a few broccoli florets.

Spoon ¾ of the alfredo sauce over the vegetables and toss well.

Pour the cheese and vegetables into the prepared casserole dish.

Nestle the reserved broccoli florets into the sauce on top of the casserole so that the green tops remain uncovered with the cheese.

Spoon the remaining alfredo sauce over the top of the casserole, taking care not to cover the broccoli on top with the sauce.

Bake the casserole in the preheated oven for 20-25 minutes until the vegetables are tender and the sauce is beginning to turn golden and bubbly.

Remove from the oven and allow it to sit for five minutes.

Serve hot.