1. Preheat your oven to 350°F (175°C). This ensures that your dish will cook evenly and the cheese will melt perfectly.
2. Prepare a baking dish by spraying it with cooking spray. This prevents the spaghetti from sticking, allowing for easy serving later.
3. In a large stockpot, fill it with about 5 quarts of water and place it on the stovetop over high heat to boil.
4. Once the water is boiling, add the spaghetti noodles. Cook them until they reach an al dente texture, typically about 9 to 10 minutes, following the package instructions for precise timing.
5. While the pasta cooks, heat 2 tablespoons of cooking oil in a skillet over medium heat. Add the diced onion, cooking until fragrant and translucent, which should take about 2 to 3 minutes.
6. Next, add the ground beef to the skillet, breaking it up with a spoon. Cook until browned and no longer pink, about 5 to 7 minutes. Once cooked, drain off any excess fat from the beef.
7. Add the taco seasoning, garlic powder, and ⅓ cup of water to the skillet. Stir to combine, allowing the water to absorb while the mixture simmers for about 2 to 3 minutes.
8. In this step, introduce the diced fire-roasted tomatoes and chopped green chilis, stirring them in along with the Velveeta and cream cheese. Turn off the heat once everything is well incorporated and creamy.
9. Once the spaghetti is ready, drain it in a colander and return it to the stockpot. Pour the meat mixture over the pasta, and stir well to ensure every noodle is coated with that savory goodness.
10. Transfer the meat and spaghetti mixture into the prepared baking dish. Sprinkle the grated Mexican cheese generously over the top, covering the dish.
11. Place the baking dish in the preheated oven and bake for 15 to 25 minutes, or until the cheese is bubbly and golden brown.
12. Once done, remove from the oven and garnish with fresh lettuce, tomatoes, and sour cream as desired. Serve hot and enjoy!