Pour 4 cups vegetable broth into large pot; add potatoes; bring to a boil and simmer until fork-tender; drain and slightly mash potatoes; reserve broth.
Pour remaining 1 cup vegetable broth into medium pot; add cauliflower and onions; bring to a boil and simmer until very soft.
Scoop cauliflower mixture into blender and process until smooth. Add reserved broth from cooked potatoes if necessary.
Blend 1/2 cup milk with brown rice flour in a bowl with a fork; set aside. Pour pureed cauliflower and remaining 1 cup milk into medium pot. Heat until warmed through, then slowly whisk in milk/rice flour mixture.
Continue whisking while heating until soup base thickens. Add reserved potatoes (and remaining broth if needed); heat until thickened and bubbly.
Serve soup into bowls garnished with 1 Tbs. shredded cheese and 1/2 slice prepared turkey bacon.