Place cauliflower on a large cutting board, stalk end down; cut lengthwise into 6 planks (about 1 inch thick).
Place cauliflower on a large baking sheet coated with cooking spray. (You will have long cross sections as well as florets.) Spray cauliflower with cooking spray. Bake at 425° for 30 minutes or until cauliflower is tender and browned, turning after 15 minutes. Remove cauliflower to a serving platter. Sprinkle with salt.
Place flour in a small saucepan. Gradually add milk and salt, stirring constantly with a whisk until blended. Place over low heat; cook until thick (about 5 minutes), stirring constantly. Remove from heat; add cheeses, stirring with a whisk until cheese melts and sauce thickens. Keep warm.
Drizzle cheese sauce over cauliflower. Sprinkle with parsley.
Lighten Up Enticement for picky eaters Although these roasted florets are fantastic on their own, no one (especially not kids) can resist them when dunked into a few spoonfuls of simple Mornay sauce. My version uses low-fat milk and a reduced amount of cheese to lower the calories.