Preheat the oven to 350F and grease a 9x13-inch baking dish.

Bring a large pot of salted water to a boil. Add the broccoli florets and blanch for 3 minutes, until bright green and just tender. Drain well and set aside.

In a large saucepan, melt 3 tablespoons of the butter over medium heat. Whisk in the flour and cook for 1 minute, until lightly golden.

Gradually whisk in the milk, a little at a time, until smooth. Cook for 4-5 minutes, whisking often, until the sauce thickens enough to coat the back of a spoon.

Remove from heat and stir in the salt, pepper, garlic powder, and nutmeg. Add 1 1/2 cups of the cheddar and the parmesan, stirring until melted and smooth.

Fold the blanched broccoli into the cheese sauce, then pour the mixture into the prepared baking dish and spread into an even layer.

Melt the remaining 1 tablespoon of butter and toss it with the crushed crackers. Sprinkle the cracker mixture over the casserole along with the remaining 1/2 cup cheddar and the turkey bacon, if using.

Bake for 25-30 minutes, until the topping is golden and the sauce is bubbling around the edges.

Let the casserole rest for 5 minutes, sprinkle with fresh parsley, and serve warm.