Liberally salt a large pot of water and bring it to a boil. Cook the spaghetti until it's al dente, approximately 6-8 minutes, then save 1 cup of the pasta water before draining. Keep the pasta warm.
In a 12-inch nonstick skillet, heat olive oil over medium-high heat. Sauté diced onion and bell pepper until they're soft and translucent, about 3 minutes.
Add garlic and cook for about 1 minute until it's fragrant.
Brown the ground beef in the skillet until it's fully cooked, roughly 5-6 minutes. Stir in spices and salt, cooking for an additional 2 minutes.
Mix in the drained Rotel tomatoes, then remove the skillet from heat. Incorporate green onions and cilantro.
Combine the cooked spaghetti with ¼ cup of the reserved pasta water, tossing to ensure even mixing. Adjust the consistency with more pasta water if needed.
Serve the taco spaghetti with your chosen taco toppings like shredded cheddar, chopped avocado, sour cream, and raw onions.