Preheat the oven to 375°F.

Spray a 9-inch square baking dish with nonstick cooking spray. Set aside.

Chop the broccoli into 1-inch florets. Fill a large pot with water and bring to a boil over high heat. Once boiling, add the broccoli florets and boil until bright green, about 1 minute. Drain the broccoli and immediately rinse under cold water. Allow broccoli to sit in the colander while preparing the sauce.

Finely mince the garlic.

In a large pot over medium heat, melt the butter. Add the garlic and sauté until fragrant, about 30 seconds. Sprinkle the flour into the pot and whisk to thoroughly combine. Continue whisking until the roux turns slightly golden and begins to smell toasty, 1-2 minutes.

Slowly stream the milk into the pot while continuing to whisk. Cook the mixture, whisking steadily, until it begins to thicken and bubble, 4-5 minutes.

Once bubbling, turn off the heat. Add the shredded cheddar, salt, and black pepper to the pot. Whisk slowly until the cheese is fully melted and the sauce is smooth.

Once the sauce is smooth, add the broccoli. Stir until the florets are thoroughly coated.

Pour the contents of the pot into the greased baking dish.

Place the bread crumbs, Parmesan, cheddar cheese, salt, and black pepper in a medium mixing bowl. Mix with a fork to combine. Add melted butter to the bowl, and stir until all the breadcrumbs are moistened. Sprinkle the crumbs in an even layer over the broccoli.

Bake the gratin on middle rack of oven until the edges are bubbling and the top is golden brown, 22-25 minutes.

Check to see that gratin is done. Remove from oven or add time as needed.

Allow the gratin to rest for 10 minutes. Plate and serve.