Prepare the Tomatoes: Bring a large pot of water to a boil.

Score a shallow "X" in the bottom of each tomato.

Boil the tomatoes for 30 to 45 seconds, then immediately transfer them to ice water.

Once cool enough to handle, peel away the skins, remove the cores, and roughly chop the tomatoes.

Cook the Aromatics: Heat the olive oil in a large Dutch oven over medium heat.

Add the onion and cook for 6 to 8 minutes, stirring occasionally until softened and translucent.

Add the garlic and cook for another 30 seconds, just until fragrant.

Add Tomato Paste: Stir in the tomato paste and cook for 2 minutes, stirring constantly until it darkens slightly and becomes fragrant.

Build the Sauce: Add the chopped tomatoes, broth, Italian seasoning, oregano, dried basil, salt, black pepper, and crushed red pepper flakes if using.

Stir well to combine.

Simmer: Bring the sauce to a gentle simmer: Reduce the heat to low and cook uncovered for 60 to 75 minutes, stirring every 10 to 15 minutes.

The sauce should reduce noticeably and become thick enough to coat the back of a spoon.

Blend (Optional): For a smooth spaghetti sauce, use an immersion blender directly in the pot or carefully transfer the sauce to a blender in batches.

For a rustic sauce, leave it chunky.

Finish the Sauce: Stir in the fresh basil: If desired, add the butter for extra richness and the balsamic vinegar for added depth.

Taste the sauce and add the sugar only if the tomatoes are noticeably acidic.

Serve immediately over hot cooked spaghetti with freshly grated Parmesan cheese and additional basil if desired.