Preheat oven to 400°, line baking sheet with aluminum foil or parchment paper (for easier clean up).
With 1 Tbsp. of olive oil brush the insides of the squash. Sprinkle with garlic pepper (or salt and pepper). Place squash face down on the prepared baking sheet and roast in the preheated oven for 30-40 minutes until tender.
While the squash is roasting. In a large frying pan over medium heat - heat remaining 1 Tbsp. olive oil and onion. Cook until onion begins to soften, add garlic and Italian sausage. Cook, stirring frequently, until the sausage is cooked through.
Add spinach, cream cheese and ½ cup of shredded mozzarella cheese. Continue cooking, stirring until the spinach is warmed and the cheese is melted.
Once squash is finished cooking, use a fork to gently mash the inside. Divide the sausage mixture evenly among the squash halves. Top each with the remaining mozzarella and the feta cheese.
Place squash back in the oven for a 3-5 minutes to melt the cheese.