Heat the oil: In a large skillet or pot over medium-high heat, add the olive oil and let it shimmer for about 1 minute.
Brown the beef: Add the ground beef and diced onions to the hot oil, breaking the beef apart with a spoon as it cooks, about 5-7 minutes until no pink remains.
Add seasonings: Stir in the taco seasoning, salt, and pepper, mixing well to coat all the beef, about 1-2 minutes.
Combine liquids and noodles: Pour in the undrained can of Rotel tomatoes and the 3 cups of water, then add the uncooked spaghetti noodles directly to the pot.
Simmer until tender: Bring the mixture to a boil, then reduce heat to medium and simmer uncovered for 12-15 minutes, stirring occasionally, until the noodles are tender and most of the liquid has been absorbed.
Remove from heat: Take the pot off the heat and let it sit for 2 minutes so the noodles can finish absorbing any remaining liquid.
Add the cheese: Sprinkle the shredded cheddar cheese over the top and stir gently until the cheese melts and coats the pasta evenly.
Garnish with cilantro: Top with fresh chopped cilantro for color, freshness, and authentic taco flavor.
Serve immediately: Transfer to serving bowls or plates and enjoy while hot, adding optional toppings like jalapeños, sour cream, or lime if desired.