Warm the olive oil in a pan over high heat until it begins to shimmer.

Introduce the diced onion and sauté, stirring often, until translucent and glossy—about three minutes. As they cook, the onions should audibly sizzle. Incorporate the garlic and stir thoroughly to prevent scorching, cooking for an additional two minutes.

Mix in the basil, red pepper flakes, salt, and sugar. Blend well. Add the butter and continue stirring for about a minute as it melts.

Pour in half of the crushed tomatoes and scrape the bottom of the pan to release any browned bits. Reduce the heat to low. Add the remaining tomatoes and stir in the measured water. If the sauce appears overly thick, gradually incorporate more water.

Allow the sauce to simmer gently for at least 10 minutes, or up to one hour. For extended cooking, stir occasionally and add water as necessary to maintain a smooth, pourable consistency.