Preheat oven to 400°F (200°C). Cut the spaghetti squash in half lengthwise and scoop out seeds.
Roast cut-side down for 45–60 minutes, checking for tenderness and making sure you can gently pull the “strands” easily from the squash skins.
Heat the black beans, corn and bell pepper mixture with chili powder, cumin, salt and pepper.
Top the roasted squash halves with this mixture.
Sprinkle with fresh cilantro and add salsa or avocado if desired.