Preheat the oven to 400°F.
Cut the spaghetti squash in half lengthwise and scoop out the seeds.
Drizzle the inside with olive oil, and sprinkle with salt and black pepper.
Place the squash cut-side down on a baking sheet and roast for 30-40 minutes, or until tender.
In a large bowl, combine the cooked quinoa, black beans, corn, diced tomatoes, chili powder, cumin, and garlic powder. Mix well to combine.
Once the squash is done roasting, carefully flip it cut-side up. Use a fork to scrape the flesh into spaghetti-like strands.
Add the strands to the quinoa mixture and stir until well combined.
Spoon the filling back into the spaghetti squash halves and top with shredded cheese.
Return to the oven and bake for an additional 10-15 minutes, or until the cheese is melted and bubbly.
Remove from the oven and let cool for a few minutes. Garnish with chopped cilantro if desired. Serve warm with a dollop of sour cream or Greek yogurt if you like.