- Preheat oven to 400°F. Spray large rimmed baking sheet with nonstick spray.
- Arrange prosciutto on prepared baking sheet in single layer. Bake until crisp, 5–6 minutes. Transfer prosciutto to plate. Maintain oven temperature. Do not wash baking sheet.
- Meanwhile, spray large skillet with nonstick spray and set over medium heat. Add garlic and pepper flakes. Cook, stirring constantly, until fragrant, about 30 seconds. Add half of kale and cook, stirring often, until wilted, about 1 minute. Add remaining kale and the water. Cook, stirring often, until kale is crisp-tender and water evaporates, about 5 minutes. Stir in salt.
- Arrange bread on same baking sheet and sprinkle with Parmesan. Top evenly with kale; sprinkle with mozzarella. Bake until cheese melts, about 5 minutes. Crumble prosciutto and sprinkle over sandwiches.
- Per serving: 1 open-face sandwich
There are two types of kale: curly kale and flat-leaf kale (also called lacinato, Tuscan, or dinosaur). Generally they are interchangeable in recipes, but flat-leaf kale is a bit more tender and take a few minutes less to cook.