Preheat your oven to 350°F.
Boil the cauliflower in a large pot of salted water until tender but firm, about 5 to 6 minutes. Drain the cauliflower well and place it in a 2-quart casserole dish.
While the cauliflower is boiling, melt the butter in a small saucepan over medium heat. Add the flour to the melted butter and cook for about 1 to 2 minutes, stirring occasionally.
Slowly add the milk, whisking constantly. The sauce should begin to thicken as you incorporate the milk.
Once the sauce has thickened, mix in the dry yellow mustard and stir to combine. Gradually add 4 ounces of the cheddar cheese, stirring until it’s completely melted and the sauce is smooth.
Pour the cheese sauce over the cauliflower in the casserole dish, ensuring it’s evenly coated. Sprinkle the remaining 4 ounces of cheddar cheese on top.
Bake for 15 minutes or until the cheese is slightly browned and bubbly.
