Cut cauliflower into bite-sized pieces. Bring a large pot of water to a boil and cook cauliflower for 4-5 minutes or until tender crisp. Drain well.
Add heavy cream, cream cheese, and butter to a saucepan and stir over medium heat until smooth and cream cheese is melted.
Add dry mustard powder, onion powder, and cayenne pepper. Bring to a boil then cook 2 minutes while whisking.
Remove from heat, stir in parmesan and 1 ¼ cups cheddar cheese. Whisk until smooth.
Toss together cauliflower and sauce in a 2-quart baking dish and top with remaining ¼ cup cheese. Bake 20-25 minutes or until top is browned and bubbly.
Notes
Shred block cheddar cheese as it will melt better than pre-shredded cheese. Pre-shredded has anti-caking agents in it that will inhibit melting.
Refrigerate leftover cauliflower casserole in a covered container for up to 4 days.
Reheat leftovers on the stovetop or in the microwave.
Freeze cooked and cooled cauliflower casserole in zippered bags for up to 1 month. The texture may not be the same, but perfect to blend into a soup.