Preheat oven to 375°.
Slice the sweet potatoes evenly, about 1/8 of an inch thick. You can use either a mandolin or a sharp knife.
In a large pan, sauté the onions and garlic in the olive oil for 2-3 minutes over medium heat, being careful not to burn the garlic. Then add the ground beef, oregano, salt and pepper. Cook until meat is well-browned, which should take about 7-8 minutes, stirring frequently. Add the tomato sauce and stir for a couple of minutes. Set aside.
In a medium mixing bowl, combine the ricotta cheese, pesto and a bit of salt to taste. Mix well and set aside.
To assemble the lasagna, use a 9x13 glass pan and create the first layer with the sweet potato slices. Make sure to overlap each slice to create an even layer.
Pour out half the meat sauce over the sweet potato slices and spread out evenly. Alternate with zucchini if using.
Pour out half the ricotta cheese mixture over the meat sauce and spread evenly.
Repeat each layer again. Then top with the grated mozzarella (optional).
Cover with tin foil and place in the oven for 45-50 minutes, or until the sweet potato slices are tender.
Let the lasagna rest for a couple of minutes when it comes out of the oven. Then, enjoy!