Preheat your oven to 400°F (200°C). Halve the spaghetti squash lengthwise and scoop out the seeds. Drizzle the cut sides with olive oil and sprinkle with salt and pepper.

Place the squash cut-side down on a rimmed baking sheet and roast it for 40–50 minutes until the skin is easily pierced and the flesh flakes into strands. Alternatively, you can microwave the halved squash for 10–12 minutes after covering it.

While the squash roasts, heat some olive oil in a skillet over medium heat. Sauté the onion and bell peppers until softened, about 5–7 minutes. Add in the garlic powder and Italian seasoning, cooking for another 30 seconds.

Once the squash is cool enough to handle, scrape out the flesh using a fork to create strands.

In a large bowl, mix the spaghetti squash strands, sautéed vegetables, diced tomatoes, salt, pepper, mozzarella, and most of the Parmesan.

Transfer the mixture to a lightly oiled baking dish. Sprinkle the remaining Parmesan on top and drizzle with more olive oil if desired.

Bake in the preheated oven at 375°F (190°C) for 15–20 minutes, until the cheese is melted and lightly golden.

Allow the dish to rest for 5 minutes before serving.