Step 1: Preheat your oven to 375°F (190°C). Prepare the zucchini by washing them thoroughly. Slice each zucchini in half lengthwise. Using a spoon, gently scoop out the flesh from the center of each zucchini half, leaving a 1/4-inch border. Discard the scooped-out flesh or save for another recipe.
Step 2: Lightly salt the inside of the zucchini boats and place them cut-side up on a baking sheet. Bake for 10-12 minutes, or until slightly tender but still firm. This par-cooking helps prevent soggy zucchini.
Step 3: While the zucchini bakes, heat the olive oil in a large skillet over medium heat. Add the chopped onion and cook for 3-5 minutes until softened. Add the sliced mushrooms and cook for another 5-7 minutes, until they release their liquid and begin to brown. Stir in the minced garlic and cook for 1 minute until fragrant.
Step 4: Add the chopped fresh spinach to the skillet and cook until it wilts completely, about 2-3 minutes. If using frozen spinach, add it and cook until any remaining liquid evaporates. Remove the skillet from the heat.
Step 5: In a medium bowl, combine the ricotta cheese, 1/4 cup of the shredded mozzarella, the beaten egg, fresh chopped basil, kosher salt, black pepper, and optional red pepper flakes. Add the cooked spinach, mushroom, and onion mixture from the skillet and stir until well combined. Taste and adjust seasonings if necessary.
Step 6: Carefully fill each par-baked zucchini boat with the ricotta-spinach-mushroom mixture, mounding it slightly. Sprinkle the remaining 1/4 cup of shredded mozzarella and the grated Parmesan cheese over the top of each boat.
Step 7: Return the baking sheet to the oven and bake for an additional 12-15 minutes, or until the zucchini is tender and the cheese is melted and lightly golden and bubbly. Serve hot and enjoy!