Preheat the oven to 375°F. Prepare a deep baking dish with a light coating of butter, olive oil, or nonstick cooking spray. Set aside.
In a small skillet, heat the olive oil over medium-high. Add the chopped garlic and mushrooms and cook, stirring occasionally, until the mushrooms release their moisture and turn golden brown, about 8-10 minutes. Remove from heat and let cool for 5 minutes.
In a large mixing bowl, combine the ground turkey, cooled mushroom mixture, breadcrumbs, and egg. Mix gently with your hands or a spatula until evenly combined—avoid overmixing to keep the meatloaf tender.
Lay a sheet of parchment paper or plastic wrap on a flat surface. Transfer the turkey mixture onto it and pat it into a 1/2-inch-thick rectangle, roughly 8x10 inches in size.
Sprinkle the mozzarella evenly over the meat mixture and layer the spinach leaves on top. Starting from one short end, carefully roll the meat mixture into a log, using the parchment to help shape it as you go. Pinch the seam and ends to seal. Transfer the rolled loaf seam-side down into the prepared baking dish.
Bake uncovered for 35 to 45 minutes, or until the internal temperature reaches 160°F. The exterior should look slightly golden, and you may notice some bubbling juices along the edges.
Remove from the oven and brush the top evenly with marinara sauce. Return to the oven and bake for another 10 minutes, until the sauce is heated and the loaf reaches 165°F.
Remove from the oven and allow the meatloaf to rest in the baking dish for about 5–10 minutes before slicing to allow the juices to redistribute.
Slice into thick portions, top with extra marinara if desired, and serve warm.