Preheat oven to 350 degrees.
Drizzle each cut side of squash with a little olive oil and sprinkle with salt and pepper.
Place cut squash halves on parchment lined baking sheet. Place in the oven and bake until soft. About 45 minutes. Then pull out of oven to stuff.
Meanwhile heat 1 tbsp olive oil over medium heat in a large skillet. Once hot and shimmering add in the onion and cook until soft about five minutes.
Add in the garlic and cook for thirty seconds, then add in the sausage. Cook until the sausage is no longer pink and cooked through. Then toss in spinach and cook until wilted.
Transfer sausage mixture to a bowl and allow to cool slightly.
Add in 1/3 C panko, 1/4 C chestnuts, and 1/4 C parmesan, 1/2 tsp kosher salt and 1/4 tsp pepper.
Stuff the stuffing into the squash halves. Bake in the oven for 15 minutes.
Mix up 2 tbsp panko, 1 tbsp parmesan, and 3 roughly chopped chestnuts together in a small bowl.
During the last five minutes top each mound of stuffing with the panko topping and drizzle with a little more olive oil. Bake for the last five minutes.