Prepare the squash: Preheat the oven to 425°F. Cut the squash in half lengthwise and remove the seeds. Brush the flesh with 2 teaspoons oil (you can eyeball this) and place the halves cut side down on an oiled baking sheet (or line with parchment paper for easy cleanup). Roast in the oven until tender, about 35 minutes. (Helpful hint: Check a few minutes early and add a few minutes if needed, as all ovens vary. The flesh should easily pull away from the shell but have a hint of firmness to it.) Remove from oven. When the squash is cool enough to handle, scrape with a fork so that it forms spaghetti-like strands. You’ll need about 6 cups (measured before removing the moisture; well filled but not packed) for the lasagna. Place the squash in a clean tea towel and wring out the excess moisture. (Helpful hint: You can use paper towels, but I find the tea towel to be far more effective, and then I shake it out and reuse it to squeeze the spinach later. Prep-ahead tip: Once the moisture is removed, you can refrigerate the squash for up to a day.)
Wilt the spinach: Heat a 12-inch skillet over medium heat. Add the spinach and cook until wilted. Remove from the pan and when cool enough to handle, squeeze out the excess liquid as done with the squash. Prefer to use the microwave? To do it this way, I use a vented bowl and high power for 60 seconds and then check and add another 15 seconds or so as needed.
Cook the turkey: Lightly oil the same skillet—you want just enough oil to lightly coat pan, about 1½ – 2 teaspoons—and heat over medium. Sauté the ground turkey, crumbling as you go. When just short of being cooked through, add the minced garlic and sauté until fragrant, about a minute. Remove from the heat and add the drained spinach, 1 teaspoon kosher salt, and freshly ground pepper to taste (I use ¼ – ½ teaspoon). Stir to evenly incorporate.
Prepare the ricotta mixture: Whisk the egg in a medium bowl with a fork, and then stir in the ricotta, and ½ teaspoon each salt, pepper, onion powder, and dried oregano. Then stir in ½ cup of the Mozzarella cheese; set aside.
Build the lasagna: Lightly coat an 8×10 or 9×9-inch baking dish with olive oil or spray, and then spread ½ cup sauce over the bottom of the dish. Top with one-third of the squash, and then spread with half of ricotta mixture. Top with half of the turkey/spinach mixture. Dollop with another ½ cup sauce—no need to spread, just dribble evenly. Top with another third of the squash; spread with remaining ricotta mixture. Top with remaining turkey/spinach mixture; top with remaining squash. Finally, spread with ¾ cup sauce. (There will be sauce remaining.)
Bake: Turn the oven down to 375℉ and place the lasagna on a rimmed baking sheet to catch any drips and to make it easier to get in and out of the oven. Bake, uncovered, for 30 minutes. Remove from the oven, and sprinkle with the remaining 1 cup of mozzarella, followed by the Parmesan cheese. Return to the oven and bake for 15 minutes more or until the edges are bubbly and the cheese is melted. If you’d like the top to have more golden-brown color, broil the casserole on the top rack for 1 to 2 minutes, watching VERY closely so as not to burn. Remove from the oven and let rest for 10-15 minutes before serving. (This will give the juices time to reabsorb and make for easier cutting.) If desired, warm the remaining marinara and serve on the side. May pass additional Parmesan cheese at the table too.