Melt butter over medium heat in a Dutch oven or large saucepan.
If you have extra time, sauté the onion in butter for several minutes. If not, just add onion wedges, water, can of tomatoes with their juices, and the salt.
Bring to a simmer. Cook, uncovered, for about 40 minutes. Stir occasionally and add additional salt as needed.
Blend the soup — it doesn’t need to be ultra-smooth, some texture is nice.