Preheat the broiler to high and position the oven rack in the top third of the oven.
Cut the broccoli crowns into evenly sized florets, roughly 1½ to 2 inches wide.
Steam the florets for 4 to 5 minutes, until just tender but still slightly firm when pierced with a fork.
While the broccoli steams, melt the butter in a small saucepan over low heat and stir in the garlic for about 30 seconds until fragrant.
Transfer the steamed broccoli to a broiler-safe dish, pour the garlic butter over it, and toss gently to coat. Season with salt and pepper.
Sprinkle the shredded cheddar evenly over the broccoli, then scatter the parmesan over the top in a loose, even layer.
Broil for 2 to 3 minutes, watching closely, until the cheese is fully melted with a few golden-brown spots.
Let rest 2 minutes before serving.