Preheat oven to 350˚F.
Add broccoli florets to a 9x13 baking dish and set aside. Melt the butter in a large skillet set over medium heat.
Add garlic to the melted butter and cook for 10 seconds, or just until fragrant.
Sprinkle the flour over the melted butter and garlic; cook and whisk for 1 minute.
While whisking, start to slowly pour in the milk; continue to cook and whisk until everything is fully combined.
Whisk in the water or chicken broth; cook for 3 to 4 minutes or until the sauce is thickened.
Remove from heat and whisk in the cheeses, salt, and pepper.
Pour the cheese mixture over the broccoli in the baking dish; stir it all together until combined. Set aside.
In a small mixing bowl combine the panko crumbs and melted butter.
Sprinkle the panko mixture over the broccoli.
Cover with foil and bake for 25 minutes.
Remove foil and continue to bake for 15 minutes, or until top is browned and the casserole is bubbly.
Remove from oven and let stand 5 to 8 minutes.
Taste for salt and pepper; adjust accordingly.