Preheat oven to 375 degrees F (190 degrees C). Grease a 2 1/2-quart casserole dish.
Heat a skillet over medium heat; cook sausage until heated through, 5 to 7 minutes. Cut sausage into small cubes.
Bring a large pot of lightly salted water to a boil. Cook elbow macaroni in the boiling water, stirring occasionally until tender yet firm to the bite, 8 minutes. Drain.
Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil. Add cauliflower, cover, and steam until tender, 5 to 6 minutes.
Melt butter in a saucepan over medium heat; add flour and cook until a thick paste forms, 1 to 2 minutes. Slowly mix milk into butter-flour mixture until sauce is smooth and thickened, 2 to 4 minutes. Stir 1 cup Cheddar cheese into sauce until melted and smooth.
Spread cauliflower into the prepared casserole dish; top with macaroni, covering the entire surface. Stir cheese sauce into cauliflower and macaroni until well coated. Sprinkle 1 cup Cheddar cheese over the top.
Bake in the preheated oven until cheese is melted and bubbling, about 35 minutes.