Cut zucchini lengthwise into 1/8" thick slices. Line large baking sheet with double layer of paper towels.
Place sliced zucchini in a single layer and sprinkle with salt, repeating with remaining zucchini. Set aside for 30 minutes. Salt will pull away moisture from zucchini making less watery zucchini lasagna.
Make meat sauce, cheese and spinach fillings:
In the meanwhile, preheat large skillet on medium-high heat, swirl oil to coat and add ground meat. Cook for 5 minutes, constantly breaking into small pieces and stirring. Add 1 tsp oregano, basil, garlic powder, salt and pepper; cook for 30 seconds, stirring.
Add tomato sauce and maple syrup; stir, bring to a boil and simmer on low for 5 minutes. Transfer to a medium bowl and set aside.
Return skillet to medium heat, add spinach in batches as it starts to wilt, stirring in between. Transfer to a bowl and let cool.
In a medium bowl, add ricotta cheese, egg and 1/2 tsp oregano. If using cheese with large curds, puree with an immersion blender until smooth. Set aside.
Assemble lasagna:
Preheat oven to 375 degrees F. Now it’s finally time to assemble zucchini lasagna.
Pat dry each zucchini layer with paper towels transferring to another dish. You want to absorb moisture between each zucchini layer as much as possible. Using your hands squeeze as much liquid as possible from spinach and set side.
In 9" x 13" baking dish, add a few tablespoons of tomato meat sauce, spread with a spoon and sprinkle with quinoa. Place sliced zucchini overlapping a bit to create a single layer. Spread 1/3 of tomato meat sauce, then 1/3 of ricotta cheese mixture, sprinkled with 1/4 of mozzarella and 1/3 of spinach.
Repeat until you run out of ingredients finishing with a layer of zucchini, topped with a bit of meat sauce, then sprinkled with remaining mozzarella and all of Parmesan cheese.
Bake covered with aluminum foil or silicone lid for 30 minutes, then uncover and bake for another 20 minutes. Let stand for 20 minutes before slicing. Cut into 8 slices and serve hot.
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Notes
Store: Refrigerate in an airtight glass container for up to 5 days. I would be hesitant to freeze it as squash consists of 90% water & isn't a good vegetable to freeze and thaw. You can try!
Make ahead: Assemble and refrigerate tightly covered with plastic for up to 24 hours. Before baking you can remove extra water accumulated at the bottom of the dish with a turkey baster. Let ceramic or glass dish warm up on the counter for about 15 minutes before baking.
More tips to make it less watery:Salting, paper towels and quinoa all help with getting rid of extra moisture. Don’t worry about liquid while baking. Once it rests for about 20 minutes, you will see there will be much less liquid left.You can also grill sliced zucchini before assembling. Just cut them a bit thicker.But overall, I personally think, it's OK to have more juices at the bottom of the dish. It makes for extra delicious sauce that doesn't make a veggie lasagna soggy like it would traditional lasagna. So I think it's even better to have more sauce at the bottom!