Preheat oven to 350 degrees. Lightly grease a large casserole dish.
Crush crackers into large crumbs in a large bag or food processor. Add melted butter a little at a time while mixing well. The crackers should be coated in butter, but not wet or drippy. Some gluten free crackers will absorb more butter than others, so you may need a little more or less than a ½ stick of butter.
In a large bowl combine broccoli, cheeses, ½ of the cracker mixture and salt & pepper to taste. Reserve the remaining cracker mixture for the topping.
Pour the broccoli mixture into a well-greased casserole dish. This can be covered and stored in the fridge for up to 24 hours. When ready to bake, return the glass or ceramic dish to room temp before baking, top with the remaining crackers and bake.
Bake for approximately 45 minutes until it is warmed through and the cheeses are melted and the topping is slightly browned. Watch to be sure the top doesn't burn, cover with foil if it starts to get too dark. Add 10-15 minutes baking time if the casserole is still cold when it goes in the oven.