Melt butter over medium heat in a Dutch oven or large saucepan
Add onion wedges, water or broth, tomatoes with their juices, and salt. Bring to a summer. Cook, uncovered for about 40 minutes. Stir occasionally and add additional salt if needed.
Blend the soup. If you use a regular blender, it’s best to blend in batches and not fill the blender as much as you would. Remove the center insert of the lid and cover it with a kitchen towel while blending – this helps release some of the steam and prevents the blender lid from popping off.
Season to taste – you can use black pepper, basil, or any other seasoning you like. Can serve with grilled cheese sandwiches, crackers, or whatever you enjoy eating soup with!