Heat the olive oil in a large Dutch oven or wide, heavy-bottomed skillet over medium-high heat. Add the ground beef and diced onion, then cook for 6 to 8 minutes, breaking the beef into small pieces, until browned and the onion is soft.

Drain excess fat if needed, then add the minced garlic. Stir and cook for about 30 seconds, just until fragrant; avoid letting the garlic darken.

Stir in the crushed tomatoes, beef broth or water, Italian seasoning, salt, black pepper, and optional red pepper flakes. Mix well so the browned meat is distributed throughout the tomato base.

Add the uncooked spaghetti and press the strands down into the liquid. Break the spaghetti in half if necessary so it fits comfortably in the pan.

Bring the mixture to a boil, then reduce the heat to a steady simmer. Cover the pan partially and cook for 15 to 18 minutes, stirring every few minutes so the pasta does not stick.

Continue cooking until the spaghetti is tender and the sauce has thickened. If the sauce becomes too thick before the pasta is done, stir in a small amount of water.

Taste and adjust the seasoning, then let the finished spaghetti rest for 2 to 3 minutes. Serve hot with grated Parmesan and chopped basil or parsley.