Preheat the oven to 400°F (200°C).
Cut the spaghetti squash in half lengthwise and scoop out the seeds.
Place the squash halves cut-side down on a baking sheet lined with parchment paper.
Bake for 35-40 minutes, or until the flesh is tender and can be shredded with a fork.
While the squash is baking, heat olive oil in a large skillet over medium heat.
Season the chicken breasts with cumin, chili powder, smoked paprika, garlic powder, onion powder, salt, and black pepper.
Add the chicken breasts to the skillet and cook for 6-7 minutes per side, or until fully cooked and no longer pink inside.
Remove the chicken from the skillet and let it rest for 5 minutes, then shred it using two forks.
In the same skillet, add chopped onion and bell peppers and sauté for 4-5 minutes until softened.
Add the diced tomatoes, black beans, and corn to the skillet and stir to combine. Cook for 2 minutes until heated through.
Remove the spaghetti squash from the oven and use a fork to shred the flesh into strands, transferring it to a large mixing bowl.
Add the shredded chicken and sautéed vegetable mixture to the bowl with the spaghetti squash. Mix well.
Add half of the shredded Mexican blend cheese and half of the cheddar cheese to the bowl and stir to combine.
Transfer the mixture to a greased 9×13-inch casserole dish and spread evenly.
Sprinkle the remaining shredded cheeses evenly over the top.
Bake the casserole at 375°F (190°C) for 10-15 minutes, or until the cheese is melted and bubbly.
Remove from the oven and sprinkle chopped fresh cilantro on top before serving.