Jumbo Lump Crab Filled Sweet Potato Cannelloni with Parmigiano Sauce

Name: Jumbo Lump Crab Filled Sweet Potato Cannelloni with Parmigiano Sauce
Ingredients: 1 Granny Smith apple, peeled and brunoised; Juice of 1/2 lemon; 1 large sweet potato, peeled; Kosher salt; 2 tablespoons butter; Pinch crushed red pepper flakes; 3 scallions, sliced; 1 tablespoon whole-grain mustard; 1 pound jumbo lump crab, picked through; Micro arugula, for garnish; Grated Parmigiano-Reggiano, for serving; Parmigiano Sauce, for serving, recipe follows; Olive oil; 1 medium onion; 4 tablespoons butter; 1/4 cup all-purpose flour; 2 cups milk; Pinch grated nutmeg; 2 cups grated Parmigiano-Reggiano; 1/2 cup creme fraiche