Preheat the oven to 400F and place the cauliflower florets on a large rimmed baking sheet.
Drizzle with 2 tablespoons of the oil and sprinkle with the salt and pepper. Toss to combine well, then roast for 15 to 20 minutes, until nicely browned.
In a large Dutch oven or stockpot, heat another tablespoon of the oil over medium heat. Add the garlic and 3 tablespoons of the chopped sage and sauté until fragrant, 1 to 2 minutes.
Add the cauliflower and the broth. Bring to a boil. Add the Parmesan rind, if using and reduce the heat to a simmer. Cook 15 minutes, then fish out the Parmesan rind.
Transfer to a blender (or use an immersion blender) and blend until smooth. Return the the pot and stir in the grated Parmesan until melted.
Heat 1 tablespoon of the oil in a medium saucepan over medium heat. Add the prosciutto and cook until almost crisp, then add the chopped sage and cook another minute. Remove to a paper towel lined plate.
Add the remaining oil to the pan. Once hot, add sage leaves to the oil and cook until crisp, about 10 seconds. Remove to the plate.
Divide the soup among 6 soup bowls and sprinkle the tops with the garnish. Add 1 sage leaf.