Add onion wedges, tomatoes and their juices, and salt. Bring to a simmer.
Cook uncovered for 40 minutes, stirring occasionally.
Blend the soup: If using an immersion blender, do this right in the pot. If using a regular blender, allow soup to cool slightly first. Then blend in small 1 cup batches.
Notes
If you have dried herbs such as oregano or thyme, those can be added to this soup as well.
This soup can be frozen for up to 3 months. To make reheating easy, freeze in zip-top bags in one-serving portions.