Preheat the oven to 375° and lightly grease an 8×8 casserole dish.
To Blanch Cauliflower:
Cut cauliflower florets off of the stem. Discard the stem. Prepare a large mixing bowl filled with cold water and ice about 2/3 of the way and set it close to the stove.
Bring a pot of water to boil and add cauliflower florets. Let them cook for about 4-5 minutes. Use a slotted spoon to take them out of the pot and right into the iced water to stop them from cooking. Le them cool completely.
Take cauliflower florets out of the iced water and into a clean towel. Pat them dry and set aside.
Prepare Cheese Sauce:
Preheat a Dutch oven or another pot over medium heat and add a tablespoon of oil. Add diced bacon and sauté until bacon bits are fully cooked and take them out. Set them aside for the topping.
Take most of the bacon grease out of the pot but leave behind about 2 tablespoons for cook with.
Sauté diced shallots until completely softened and starts to get golden brown.
Sprinkle the flour over the shallots and whisking constantly. Once incorporated, start SLOWLY pouring in vegetable stock and then milk, while constantly but slowly whisking.
Stir in Dijon mustard, salt and pepper. Start adding shredded cheddar, shredded Swiss, and Blue cheese crumbles a handful at a time, while whisking slowly and constantly. Keep stirring until cheese is melted and smooth.
Once the cheese sauce is thickened, and blanched cauliflower florets. Mix all well and transfer to the casserole dish.
Baking Cauliflower Casserole:
In a small bowl, combine the bacon bits, bread crumbs, and melted butter. Mix very well and spread the mixture over the casserole evenly.
Bake the casserole for about 20 minutes, until bubbly around the sides.