Preheat oven to 375 degrees. Place riced cauliflower in a big mixing bowl. Melt butter in medium heavy-duty saucepan. Add diced onion and sauté over medium heat. Turn heat down to low. Add flour, stir until lumps are out. Stir in whipping cream and sour cream. Once all stirred in add cream cheese, stirring until all melted. Pour over riced cauliflower, add sea salt, and mix until well combined. Place mixture in a 13×9 baking pan. Sprinkle top with shredded cheese. Bake for 45-50 minutes.