Preheat the oven to 400 degrees. Coat an 8 x 8 baking dish with cooking spray.
Combine the panko crumbs, parmesan cheese and 1 1/2 tablespoons of melted butter then toss to coat evenly; set aside.
Place the broccoli florets in a microwave safe bowl then season with a small touch of sea salt, to taste. Cover the bowl and cook in 1 minute increments until just fork tender--the broccoli will cook more in the oven. Side Note: My broccoli took 1 minute 30 seconds but it really depends on the microwave and how big the florets are.
Place the cooked broccoli in the prepared baking dish; set aside.
Heat the remaining 1 1/2 tablespoons of butter in a skillet over medium-high heat. Add the shallots and cook, stirring often, for 2 minutes.
Add the garlic and cook, stirring constantly, for 1 minute.
Add the flour and cook, stirring constantly, for 1 minute.
Slowly whisk in the half-and-half and bring to a boil.
Off heat very slowly add the cheddar cheese while whisking so it melts evenly.
Add the American cheese, hot sauce and dried mustard and whisk until combined. Taste and season with sea salt and freshly cracked pepper, to taste.
Pour the cheese sauce on top of the broccoli then toss to coat evenly.
Sprinkle the prepared panko crumbs on top evenly.
Bake for 10-15 minutes, until the casserole is bubbling around the edges and golden brown on top. Let cool for 5-10 minutes before serving. Enjoy!