Warm the oil: Heat 1 tablespoon of the olive oil in a large heavy-bottomed saucepan over medium heat. Add the diced onion and cook for 5 to 7 minutes, stirring occasionally, until softened and translucent. It should not be deeply browned.

Cook the garlic: Add the minced garlic and cook for about 30 seconds, stirring constantly, until fragrant. Do not let the garlic turn dark brown, as it can become bitter.

Toast the tomato paste: Stir in the tomato paste and cook for 1 to 2 minutes. The paste should darken slightly and coat the bottom of the pan.

Add the tomatoes and seasonings: Stir in the crushed tomatoes, Italian seasoning, dried oregano, granulated sugar, kosher salt, black pepper, optional red pepper flakes, and water. Scrape the bottom of the pan with a wooden spoon to loosen any concentrated tomato paste.

Bring to a gentle simmer: Increase the heat slightly and bring the sauce to a gentle simmer. Once bubbling, reduce the heat to low. The surface should show occasional small bubbles rather than a vigorous boil.

Simmer the sauce: Cook uncovered for 35 to 40 minutes, stirring every 5 to 10 minutes. As it reduces, the sauce should become thicker and darker, with the oil lightly visible around the edges.

Adjust the texture: Taste the sauce and adjust the seasoning only if needed. For a smoother sauce, use an immersion blender directly in the saucepan until it reaches your preferred consistency. Leave it chunky if you prefer a more rustic sauce.

Finish with olive oil and basil: Stir in the remaining 1 tablespoon olive oil and the optional chopped fresh basil. Simmer for 1 additional minute, then remove from the heat.

Rest before serving: Let the sauce stand for 5 minutes before tossing it with cooked spaghetti or using it in another recipe. If the sauce becomes thicker than desired, stir in a small amount of warm water until it reaches the consistency you want.