Preheat your oven to 400°F and line a large baking sheet with parchment paper.
Slice the zucchini lengthwise into thin strips using a mandoline or sharp knife, then arrange them on the baking sheet along with the sliced mushrooms.
Roast the zucchini and mushrooms for 10 minutes to release excess moisture, then remove from the oven and pat dry with a paper towel.
In a medium bowl, whisk the egg and then stir in the ricotta cheese, minced garlic, dried oregano, sea salt, and black pepper until the mixture is smooth.
Spread a thin layer of marinara sauce on the bottom of a small oven-safe baking dish and top with a single layer of roasted zucchini strips.
Spread half of the ricotta mixture over the zucchini, followed by a layer of fresh spinach and another drizzle of marinara sauce.
Repeat the layers with the remaining zucchini and ricotta, finishing with the rest of the marinara, shredded mozzarella, and grated parmesan.
Bake for 20-25 minutes until the cheese is melted and bubbling, then let the lasagna rest for 5 minutes before serving to allow the layers to set.