Brown the ground beef together with the diced onion in a spacious skillet set over medium-high heat, ensuring to crumble the meat as it cooks. Continue until the beef loses its pink hue and the onions have softened, then drain any surplus fat. Incorporate the taco seasoning, diced tomatoes with green chilies, uncooked spaghetti noodles, and water. Elevate the heat to bring the contents to a vigorous boil, then cover, lower the flame, and simmer gently for 15 minutes or until the pasta is fully tender. Fold in half of the shredded cheddar, cover once more, and allow it to rest briefly until the cheese has thoroughly melted, about one minute. Remove from heat, portion onto plates, and crown each serving with the remaining cheese and a scattering of fresh cilantro. Serve immediately.