1 tablespoon olive oil
2 sweet potatoes, peeled and diced
Salt and freshly ground black pepper
3/4 cup water
1/4 cup butter
1/4 cup all-purpose flour
4 cups whole milk
1/4 tsp. nutmeg
1/4 cup dried basil leaves
12 no-boil lasagna noodles
3 cups shredded mozzarella cheese
1/4 cup grated Parmesan
Heat oil in a heavy large pot over medium-high heat. Add the sweet potato and toss to coat. Season with salt and pepper, add water and cook until tender. Remove from heat and puree in blender and/or mixer.
Melt 1/4 cup of butter in a heavy pan over medium heat, add flour and blend well. Gradually add milk. Bring to a low boil, turn down heat and simmer until sauce thickens. Add 1/4 cup parmesan cheese, blend well. Add basil, salt and pepper (to taste) and remove from heat.
Preheat oven to 375 degrees.
Butter a 13 by 9 by 2-inch glass baking dish. Spread a thin layer (about 3/4 cup) of the sauce on bottom of baking dish. Arrange 4 lasagna noodles on top of sauce. Spread 1/3 of potato over the noodles. Sprinkle with mozzarella cheese. Repeat layers, ending with lasagna noodles on top. Pour remainder of sauce over the noodles.
Cover with aluminum foil and bake for 40 minutes. Remove foil, sprinkle the rest of the mozzarella cheese on top and bake an additional 15-20 minutes or until top is golden brown.