From our friends at http://thefoodieandthefamily.blogspot.com/!
Preheat oven to 425°F. Arrange slices of zucchini in a single layer on 2 baking sheets that have been oiled with 1 teaspoon of olive oil each. Bake each sheet for 5 minutes, flip zucchini and bake for another 3-5 minutes, until zucchini is tender. Cool while you prepare the filling.
Reduce oven to 350°F.
In a large frying pan, heat remaining 2 teaspoons olive oil over medium heat. Saute the ground beef and onions until the meat is browned and the onions are translucent, about 5 minutes. Remove from heat, drain any fat, and cool.
In a large bowl, combine ricotta cheese, spinach, 1 cup of mozzarella cheese, 1/4 cup of parmesan cheese, parsley, salt, pepper, and garlic. Stir in cooled meat mixture and thoroughly combine.
Spread 1 1/2 cups of sauce over the bottom of a 9×13″ baking dish. Scoop 1-2 tablespoons of filling onto the end of each slice of zucchini. Roll the zucchini over the filling and place, seam side down, in the baking dish. Arrange the rolled zucchini tightly in the pan until the pan is full. Sprinkle with any leftover filling and cover with remaining 1 1/2 cups of sauce. Top with remaining 1 cup of mozzarella and 1/4 cup of parmesan cheese.
Bake, uncovered, until heated through and the sauce is bubbling, about 30-40 minutes.