Preheat your oven to 400°F (200°C).
Cut the spaghetti squash in half lengthwise and scoop out the seeds.
Drizzle olive oil over the cut sides and season with salt and pepper.
Place the squash cut-side down on a baking sheet lined with parchment paper.
Bake for 30-40 minutes until tender.
Remove from the oven and let cool for a few minutes.
Use a fork to scrape the inside of the squash to create strands.
In a skillet, sauté the onions and bell peppers until soft.
Add diced tomatoes and cooked spaghetti squash strands to the skillet.
Toss everything together and garnish with fresh basil. Serve warm.