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. If you use a regular blender, it is best to blend in batches and fill the blender less than you usually would since the soup is so hot. We like to remove the lid’s center insert and cover it with a kitchen towel while blending — this helps release some of the steam and prevents the blender lid from popping off (which can be a big, hot mess).