Melt the butter over medium heat in a Dutch oven or large saucepan.
Add onion wedges, water, tomatoes with their juices, and ½ teaspoon of salt.
Bring to a simmer. Cook, uncovered, for about 40 minutes. Stir occasionally and add additional salt as needed.
Blend the soup, and then season to taste. The soup doesn’t need to be ultra-smooth; some texture is nice. An immersion blender does make quick work of this, or you can use a blender. When using a regular blender, work in smaller batches, and don’t fill it too high – hot soup expands! For safety, remove the lid’s center insert and cover the opening with a kitchen towel while blending. This releases steam and prevents messy splatters.
