If you don't have panko you can use regular bread crumbs instead. Slideshow: More Macaroni and Cheese Recipes
Preheat the oven to 375°F.
While the pasta is cooking, in a medium saucepan, heat the oil over medium high heat until hot, then stir in the sweet potato and cook, stirring occasionally, until golden and crisp tender, about 10 minutes.
Whisk in the flour and cook, whisking, until the flour is fragrant, about 3 minutes. Whisk in the milk and 1/2 teaspoon each salt and pepper, then bring to a boil, whisking. Boil until the milk is slightly thickened, about 8 minutes. Remove from the heat and whisk in the cheeses until melted, then whisk in the spinach. Season the sauce with salt and pepper to taste. Toss the pasta together with the sauce and transfer to a 3-quart baking dish. Sprinkle evenly with the breadcrumbs.
Bake the mac and cheese until the filling is bubbling, 20 to 25 minutes. Serve.