Prepare the broccoli. Preheat your oven, then use a large pot of well salted boiling water to blanch your broccoli for 2-3 minutes or until it’s barely tender. Drain well and set aside.
Cook. Melt some butter in a large skillet and saute the onion and cook until translucent. Add the bacon and cook until crisp. Add the garlic and cook for 30 seconds.
Prepare the sauce. Add the heavy cream to the garlic mixture and mix to combine. Add the shredded mozzarella and Parmesan. Cook, while giving the sauce an occasional stir, until the sauce has thickened.
Finishing up. Add the broccoli to a casserole dish and then pour the bacon and cheese mixture over top of it. Toss to coat and then sprinkle with extra shredded cheddar cheese.