Heat oven to 425°F. Toss cauliflower with oil and its 1/2 teaspoon salt; roast on sheet 20 to 25 minutes until browned and tender.
Meanwhile, cook macaroni in large pot with tablespoon salt until just tender; drain. Grease 2-quart dish.
Melt butter in saucepan; whisk flour 1 minute and gradually add milk. Add sauce salt and pepper; simmer 3 minutes. Remove from heat; add cheddar.
Fold pasta and cauliflower into sauce; transfer to dish. Add Parmesan; reduce oven to 375°F.
Bake 15 to 20 minutes until bubbling with lightly golden edges. Rest 5 minutes.