Warm the pot: Set a large, heavy-bottomed pot or Dutch oven over medium-high heat.
Add olive oil and let it shimmer.
Brown the meat: Add ground beef. Season with a pinch of salt and pepper. Cook, breaking it up with a spoon, until browned with some crispy bits, about 5–7 minutes.
Spoon off excess fat if there’s more than a tablespoon left.
Sauté aromatics: Add the chopped onion. Cook until softened and lightly golden, 3–4 minutes. Stir in garlic and tomato paste; cook 1 minute until fragrant and the paste darkens slightly.
Season the base: Sprinkle in oregano, basil, and crushed red pepper flakes.
Stir to bloom the spices for 30 seconds.
Add liquids and sauce: Pour in the marinara and 3.5 cups of broth (or water). Stir well, scraping up any browned bits from the bottom of the pot. Bring to a steady simmer.
Add the spaghetti: Break the spaghetti in half and scatter it into the pot in a crisscross pattern to prevent clumping.
Press the noodles down with a spoon to submerge them.
Simmer and stir: Reduce heat to medium. Cook uncovered, stirring every 1–2 minutes to keep noodles from sticking. Adjust heat to maintain a lively simmer.
If the liquid reduces too quickly before pasta is tender, add the remaining broth 1/4 cup at a time.
Finish the sauce: When the spaghetti is al dente (10–12 minutes total), taste and adjust salt. Stir in sugar if the sauce tastes too sharp. Add Parmesan and, if using, a tablespoon of butter or a drizzle of olive oil for a glossy finish.
Rest and garnish: Turn off the heat and let the pot sit 2 minutes to thicken.
Top with chopped parsley or basil. Serve hot with extra Parmesan.