How to Make It
Place an oven rack in middle position; preheat to 400ºF. Line a baking sheet with foil. Brush insides of squash with 1 Tbsp. olive oil. Sprinkle with salt. Place, cut side down, on baking sheet; bake until tender, 30 to 45 minutes.
Warm 1 Tbsp. oil in a large skillet over medium heat. Cook onion, stirring, until softened, about 3 minutes. Add sausage; cook, breaking into small pieces, until lightly browned, 5 to 6 minutes. Stir in garlic and spinach; sauté until heated through, 1 to 2 minutes.
Preheat broiler to high. Flip squash halves over and fill with sausage mixture. In a small bowl, mix Parmesan, panko and remaining 1 Tbsp. oil. Sprinkle over squash. Broil until topping is golden, 1 to 2 minutes. Serve.