Prep the squash: Preheat your oven to 400°F (200°C). Carefully cut the spaghetti squash in half lengthwise and scoop out the seeds. Rub the cut sides with olive oil and season with salt and pepper.
Roast until tender: Place the squash halves cut-side down on a parchment-lined baking sheet.
Roast for 35–45 minutes, or until the strands pull away easily with a fork and the flesh is tender but not mushy.
Cook the bacon: While the squash roasts, cook bacon in a skillet over medium heat until crispy. Drain on paper towels and crumble. Reserve 1 teaspoon of bacon fat if you’d like extra flavor in the sauce.
Make the creamy ranch base: In a large bowl, whisk together cream cheese, sour cream, heavy cream, ranch seasoning, minced garlic, and a pinch of black pepper.
If the mixture is too thick, warm it slightly or add 1–2 tablespoons more cream. Stir in the green onions, and optional smoked paprika or red pepper flakes for a little heat.
Add chicken and cheese: Fold in the shredded chicken, half of the cheddar, and half of the mozzarella. Mix until everything is coated in the ranch mixture.
Add the reserved teaspoon of bacon fat if using.
Shred the squash: When the spaghetti squash is cool enough to handle, use a fork to scrape out the strands. Pat gently with paper towels if very watery. Measure about 5–6 cups of strands.
Combine: Add the squash strands and most of the crumbled bacon to the ranch-chicken mixture.
Toss to coat evenly. Taste and adjust salt and pepper—remember ranch seasoning is salty, so go light.
Assemble and bake: Reduce oven to 375°F (190°C). Spoon the mixture back into the squash shells or into a greased 9x13-inch baking dish.
Top with the remaining cheddar and mozzarella.
Bake and broil: Bake for 12–15 minutes, until hot and the cheese is melted. Switch to broil for 1–3 minutes to brown the top. Keep a close eye so it doesn’t burn.
Finish and serve: Top with the remaining bacon and extra green onions.
Let it sit 5 minutes before serving to set. Garnish with fresh parsley if you like.