Preheat your oven to 350°F (175°C).
In a large skillet, heat the olive oil over medium heat. Add the diced onion and green bell pepper, cooking until softened, about 5 minutes.
Stir in the cooked chicken, diced tomatoes, barbecue sauce, garlic powder, onion powder, salt, and black pepper. Cook for another 3-4 minutes until heated through.
In a large mixing bowl, combine the cooked spaghetti with the chicken mixture. Mix well to ensure the spaghetti is evenly coated.
Transfer the mixture to a greased 9x13 inch baking dish. Top with shredded Monterey Jack cheese.
Bake in the preheated oven for 20-25 minutes, or until the cheese is melted and bubbly.
Remove from the oven and let it cool for a few minutes. Garnish with fresh cilantro if desired before serving.