Boil the pasta water. Bring a large pot of salted water to a rolling boil.
Use about 1 tablespoon of salt per 4 quarts of water for properly seasoned pasta.
Brown the meat. In a large skillet or Dutch oven over medium-high heat, add the ground beef. If your pan is dry, add a little olive oil. Cook, breaking it into small pieces, until deeply browned and no pink remains, about 6–8 minutes.
Season lightly with salt and pepper.
Drain excess fat (if needed). If there’s more than a tablespoon of fat left, spoon some off. Leave a bit for flavor.
Sauté onion and garlic. Push the meat to the sides and add the onion to the center. Cook until soft and translucent, 4–5 minutes.
Stir in the garlic and cook 30–60 seconds until fragrant. Don’t let it burn.
Build the sauce. Stir in the tomato paste and cook for 1 minute to caramelize it slightly. Add crushed tomatoes, oregano, basil, red pepper flakes, sugar, bay leaf, and a pinch of salt and pepper.
Add the broth or water if the sauce seems very thick.
Simmer gently. Reduce heat to low and simmer uncovered for 15–25 minutes, stirring occasionally. You want a gently bubbling sauce that thickens slightly. Taste and adjust seasoning.
Cook the spaghetti. Add spaghetti to the boiling water and cook until just shy of al dente (usually 1 minute less than package directions).
Reserve about 1 cup of pasta water before draining.
Marry pasta and sauce. Remove the bay leaf. Add the butter to the sauce and stir until melted for a glossy finish. Add the drained spaghetti and a splash of reserved pasta water.
Toss over low heat for 1–2 minutes until the sauce clings. Add more pasta water as needed.
Finish and serve. Sprinkle with chopped parsley or basil. Serve hot with plenty of freshly grated Parmesan.