Cook the Spaghetti:

Bring a large pot of salted water to a boil. Cook the spaghetti according to package instructions until al dente. Drain and set aside.

Prepare the Taco Meat:

Heat a large skillet over medium-high heat. Add the ground beef, breaking it up with a spoon as it cooks.

Once the beef is about halfway cooked, add the chopped onion and minced garlic. Continue cooking until the beef is fully browned and the onions are soft.

Stir in the taco seasoning, diced tomatoes, and water. Reduce heat to low and simmer for about 5 minutes, allowing the flavors to meld together.

Combine Spaghetti and Taco Meat:

Add the drained spaghetti to the skillet with the taco meat. Toss well to ensure the spaghetti is evenly coated with the taco sauce.

Add Cheeses:

Sprinkle the cheddar and Monterey Jack cheeses over the top of the spaghetti. Cover the skillet with a lid or aluminum foil, allowing the heat to melt the cheese.

Garnish and Serve:

Once the cheese is melted, remove from heat. Dollop sour cream on top, and sprinkle with sliced black olives, chopped cilantro, and jalapeño slices if using.

Serve hot, allowing guests to mix the creamy, cheesy top layer into the spicy spaghetti as they dish it out.