Step 1
Preheat oven to 400°F (200°C). Line baking sheet with parchment paper.
Step 2
Cut squash in half lengthwise and remove seeds. Place cut side down on prepared baking sheet. Roast for about 45 minutes or until knife easily inserts into squash.
Step 3
Remove from oven and let cool slightly. Using a fork, scrape out squash into 13 x 9 inch (3 L) casserole dish. Stir in 2 tbsp (30 mL) of the butter, 1/4 tsp (1 mL) each of the salt and pepper; set aside.
Step 4
In a large saucepan, heat remaining butter over medium heat and cook garlic, onion, oregano and remaining salt and pepper for about 5 minutes or until softened. Add chicken and stir to coat and brown all over. Stir in tomato sauce and nacho slices; bring to a simmer. Continue to simmer for 10 to 15 minutes or until thickened slightly.
Step 5
Pour chicken enchilada sauce over spaghetti squash evenly and sprinkle with cheese. Bake for 15 to 20 minutes or until cheese is melted and golden and squash is heated through. Let stand 5 minutes before serving. Serve with guacamole and sour cream, if desired.