In a large skillet, bring 1/4 cup water to a boil over medium-high heat. Add half of the spinach and cook, turning, until wilted, 1 to 2 minutes; drain. Repeat with the remaining spinach. Let cool slightly, then squeeze to remove as much moisture as possible. Coarsely chop, then transfer to a large bowl.
In the same skillet, cook the sausage over medium-high heat, breaking it up, until browned, about 5 minutes. Add 1 tablespoon olive oil and the onion and cook, scraping up any browned bits, until the onion is softened, about 5 minutes. Transfer to the spinach mixture. Stir in the mozzarella and parmesan; season with salt and pepper.
Preheat the oven to 425 degrees . Grease a baking sheet with 1 1/2 teaspoons olive oil. On the baking sheet, stretch out the dough to form a 10-by-15-inch rectangle. Mound the filling lengthwise down the center of the dough, leaving a 1-inch border on the short edges. Fold the 1-inch edges toward the center, then fold each long side over the filling. Flip the calzone seam side down and re-tuck any open ends. Rub the remaining 1 1/2 teaspoons olive oil over the calzone, then cut 2 small vents in the top. Bake until golden, 30 to 35 minutes. Serve with the marinara sauce.