Preheat oven to 375 F.
Steam the cauliflower florets until soft, about 8 minutes. Remove from heat and set aside.
Heat the Butter and oil in a large pot over medium-high heat until butter is melted
Stir in the flour to make a roux, reduce the heat to medium and stir constantly for 3-5 minutes to cook the flour through.
Slowly add the milk to the roux,and add the salt and pepper and cayenne. Continue to stir constantly over medium heat, until the mixture thickens -- another 5-10 minutes.
Remove from heat and stir in the cheese until melted. (Optionally also stir in 1 cup of lobster meat for lobster mac-and-cheese!)
Add the steamed caulflower to the cheese sauce and carefly fold together so as not to mash the cauliflower.
Pour into a 7" x 10" cassarole pan, sprinkle breadcrumbs over the top (if using), and bake for 20 minutes at 375F. Finish in the broiler for 5 minutes to crisp up the top. Serve warm!