Season a large stockpot of water generously with salt and bring to a boil. Add spaghetti and cook until al dente, about 6-8 minutes. Before draining, reserve 1 cup of pasta water. Reserve and hold warm.
Meanwhile, pour olive oil into a 12-inch nonstick skillet and heat over medium high heat. Add diced onion and bell pepper, cooking until softened and translucent, about 3 minutes.
Add garlic and cook until fragrant, about 1 minute.
Add ground beef to the skillet and brown until cooked through, about 5-6 minutes. Add spices and salt, cooking an additional 2 minutes.
Pour in drained Rotel tomatoes and stir together. Remove from heat. Toss with green onions and cilantro.
Add in cooked spaghetti along with 1/4 cup of reserved pasta water and ross until everything is mixed evenly. The taco mixture should coat and cling to pasta. If it looks dry, add a little more pasta water.
Serve with your favorite taco bar toppings -- Shredded cheddar, chopped avocado, a dollop of sour cream, raw onions.