Preheat your oven to 350°F (175°C).
In a large skillet, heat the olive oil over medium heat. Add the diced chicken and season with garlic powder, onion powder, smoked paprika, salt, and black pepper. Cook until the chicken is browned and cooked through, about 5-7 minutes.
In a large bowl, combine the cooked spaghetti, cooked chicken, barbecue sauce, diced tomatoes, and half of the green onions. Mix well until everything is evenly coated.
Transfer the mixture to a greased 9x13-inch baking dish. Sprinkle the shredded Monterey Jack cheese evenly over the top.
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 the remaining green onions and chopped cilantro, if using.