Pre-heat oven to 350°F
In a large bowl, toss broccoli florets in olive oil, 1/2 tsp salt and minced garlic.
Spread broccoli on a greased baking sheet (line with aluminum foil for easy cleanup). They can be crowded together but not overlapped on top of each other.
Bake for 15 minutes. Remove and transfer broccoli into a 12' oval baking dish (or 9' x 9' square baking dish).
Pre-heat oven to 425°F.
Bring a small pot to medium low heat on the stovetop. Add butter and flour. Mix to combine. Then add milk.
Bring to a boil and whisk continuously until flour clumps disappear. When sauce thicken slightly, add 1/2 tsp salt and 1/4 tsp black pepper. Stir to combine, then remove pot from heat.
Pour sauce and spread all over broccoli in the casserole, then top with breadcrumbs and parmesan cheese.
Bake Broccoli au Gratin in the oven for 20 minutes, then broil for 3 minutes, or until the top is slightly golden brown. Remove from oven.
Serve and enjoy!