Preheat oven to 350° and spray a 9x13 baking dish with non stick spray. Set aside.
Mix the ricotta cheese, egg, and parsley until completely combined.
In a large pan, add olive oil and heat over medium heat.
Add ground beef and onions and cook until meat is completely browned and onions are translucent.
Add garlic and sauté for an additional 1 too 2 minutes. Drain the grease and return it to pan.
Pour tomato sauce, tomato paste, crushed tomatoes, fennel seeds, oregano, basil, salt, and pepper into pan, stir.
Bring to a boil over medium heat, then reduce heat to low. Cover and simmer for 20 - 25 minutes, stirring occasionally. If the sauce is too thick, add a little water, about a tablespoon at a time to thin it out slightly.
Add ¼ of the meat mixture to the bottom of the dish, evenly covering the bottom.
Place ⅓ of the zucchini slices over the meat mixture.
Spread ⅓ of the ricotta mixture over the zucchini slices.
Spread ⅓ of the mozzarella cheese over the ricotta.
Sprinkle with ½ of the parmesan cheese.
Add another layer of meat, zucchini, ricotta, mozzarella, and parmesan, followed by a final layer of meat, zucchini, ricotta, and mozzarella.
Cover with foil and bake for 40 minutes.
Remove foil and bake for an additional 10 minutes.
Carefully remove the zucchini lasagna from the oven and let sit for 10 minutes before serving.