Combine San Marzano tomatoes, butter, yellow onion (cut-side down), water, salt, and black pepper in a medium saucepan. Heat over medium heat until the tomatoes begin to bubble.

Reduce heat to low and simmer, uncovered, stirring occasionally (about every 10-15 minutes).

Continue to cook until the sauce has thickened and the butter has separated from the liquid, forming a rich, oily sauce (about 35-45 minutes).

Remove and discard the onion half.

For a smoother sauce, use an immersion blender to blend until your desired consistency.

Serve immediately over your favorite pasta or use as a base for other dishes.