After the cauliflower has baked almost 40 minutes, start the cheese sauce.
Preheat oven to 400°. Trim bottom leaves from cauliflower. Remove core, leaving head intact. Place cauliflower on a large piece of heavy-duty aluminum foil. Loosely fold sides of foil up and over cauliflower; crimp sides of foil together to seal, leaving top of foil open. Sprinkle 2 Tbsp. water over cauliflower. Crimp top of foil together to seal. (Foil should be loose, allowing cauliflower to steam.) Place foil-covered cauliflower in a roasting pan.
Bake at 400° for 45 minutes.
Meanwhile, melt butter in a medium saucepan over medium heat; gradually whisk in flour until a paste forms, and cook, whisking constantly, 1 minute. Whisk in evaporated milk, and cook, whisking constantly, 5 minutes or until thickened and smooth. Whisk in cheese until melted. Whisk in tomatoes, 1/2 tsp. salt, and pepper.
Remove cauliflower from oven and fold foil halfway down sides of cauliflower. Top cauliflower with cheese sauce, and sprinkle with paprika and salt to taste.
Bake at 400° for 10 minutes. Serve immediately.