1. Heat oven to 350ºF.
2. Use pulsing action of food processor to process cauliflower until chopped to pea-size pieces. (Some recognizable small florets may still remain.) Place in large microwaveable bowl; stir in water.
3. Microwave on HIGH 3 min.; stir. If necessary, continue microwaving 2 min. or until any larger cauliflower pieces are crisp-tender. Drain cauliflower.
4. Cook bacon in medium cast-iron skillet on medium heat until crisp. Remove bacon from skillet; drain on paper towels. Discard all but 1 Tbsp. drippings from skillet.
5. Add yellow onions to reserved drippings; cook 3 to 4 min. or until crisp-tender, stirring frequently. Add cream cheese and hot sauce; cook and stir 2 to 3 min. or until cream cheese is completely melted and sauce is well blended. Stir in cauliflower, mayo and 1-1/2 cups shredded cheese; smooth top.
6. Crumble bacon; sprinkle over dip. Top with remaining shredded cheese.
7. Bake 25 to 30 min. or until dip is heated through and top is golden brown. Sprinkle with green onions.