Preheat oven to 350 degrees.
In a large saucepan melt butter on medium low heat. Sauté garlic in butter until it just starts to turn golden, about 2-3 minutes.
Quickly add flour, dried mustard, and black pepper, whisking continually. Continue whisking until a deep golden thick mixture is formed, about 2-3 minutes.
Slowly whisk in the milk. Return to medium-low heat, bring to a near boil, remove from heat, and then stir in cheese quickly. Continue whisking until your cheese sauce is smooth and creamy. Let cool.
While your cheese sauce is cooling, bring 6 cups of water to a boil in a medium pot. Once boiling, remove from heat and toss your potato slices into the pot, making sure they are all completely covered with water. Let this sit for 10 minutes, then pour water and soaked potatoes into a strainer. Rinse potatoes with cool water until potatoes are nice and cold. Set aside for 5 minutes.
Add 1 1/2 cups of water to cheese sauce. Then add soaked potatoes and broccoli and mix it all up well.
Add mixture to a 9x13 baking dish and cook for 50-60 minutes or until a fork can be easily inserted in the dish and the top is starting to turn golden and bubble.
If adding the toppings, add now, and continue to bake the dish for an additional 10 minutes or until golden. Serve hot.
Follow steps 1-5. Then combine cheese sauce with soaked potatoes and broccoli in a bowl, mix well. Add mixture to a one-gallon freezer safe bag, remove all air, seal, label and freeze.
For toppings (optional): Combine all ingredients in a one-quart freezer bag. Seal and shake to mix well. Open and reseal, pushing all the excess air out. Freeze flat.
The day before cooking, allow freezer bags to thaw for 24 hours in your refrigerator. On cooking day, preheat oven to 350 degrees. Then add in 1 1/2 cups water to your broccoli cheese meal freezer bag, seal and swish it around until it’s all incorporated well. Then follow steps 7-8.