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. This process helps to easily remove the skins, resulting in a smoother sauce.

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. Cooking the aromatics properly builds a flavorful base for the sauce.

Add Tomato Paste: Stir in the tomato paste and cook for 2 minutes, stirring constantly until it darkens slightly and becomes fragrant. This step enhances the sauce's depth and richness, contributing to its overall flavor.

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. This mixture is where all the flavors come together, creating the heart of the sauce.

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. This slow cooking allows the flavors to concentrate and develop.

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. Blending is optional based on your texture preference.

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. This final step enhances the flavor and balances the acidity.

Serve: Serve immediately over hot cooked spaghetti with freshly grated Parmesan cheese and additional basil if desired. Serving it hot ensures the best flavor and texture.