Prepare the sweet potatoes by peeling them and slicing into thin disks about 2 mm thick. They must be thin, otherwise, they will not cook through. Set aside while you prepare the rest of the ingredients.
To make the meat sauce, heat the oil in a pan over medium heat. Add finely chopped garlic and onion. Sauté for about 5 minutes with a bit of salt, then add the bell pepper and carrot and continue sautéing until the vegetables soften.
Add the ground meat to the pan and cook for a few minutes while stirring. Season with salt, oregano, pepper, and add the bay leaf.
Stir the tomato passata into the meat mixture, reduce the heat, and cook for about 10 minutes, stirring occasionally. Remove the bay leaf at the end.
For the bechamel sauce, melt the butter in another pan over low heat. Add flour and nutmeg, and stir for about 2-3 minutes. Gradually add the milk while continuously stirring to avoid lumps. Season with salt, nutmeg, and black pepper. Cook on low heat until the sauce thickens.
Take a baking dish and arrange a layer of sweet potatoes at the bottom as the base. Evenly spread a layer of bechamel sauce over the sweet potatoes, then add the meat and cheese.
Repeat the layering until all ingredients are used up, finishing with a layer of bechamel sauce on top. Sprinkle cheese over the final layer of bechamel sauce.
Bake the lasagna in a preheated oven at 400°F (200 °C) for about 45-50 minutes or until the sweet potatoes are tender, and the cheese on top is brown. If the cheese starts to brown too much, lightly place aluminum foil or parchment paper on top. Do not cover the lasagna completely, just lightly cover.
Let the lasagna cool before cutting and serving, at least 20 minutes.