Preheat oven to 400 degrees F (200 degrees C). Grease a baking sheet.
Place squash skin side down on prepared baking sheet. Bake until cooked through, about 1 hour. Remove from oven; cool for 10 minutes. Once squash is cool enough to handle, scrape flesh into string-like strands with a fork. Place in a bowl and set aside.
Melt 1 tablespoon of butter in a large skillet over medium-high heat. Add onion; cook and stir until onion begins to turn translucent. Stir in kale and mushrooms; reduce heat to medium low.
Stir in squash, remaining 2 tablespoons butter, garlic salt, Italian seasoning, and red pepper flakes; cook for 2 minutes.
Remove from stove and place squash mixture in a large bowl.
Stir olive oil and pesto into the squash mixture. Slowly add grated Parmesan cheese, stirring until evenly mixed.
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