Add the olive oil to a large saucepan over medium high heat.
Saute the onion in the olive oil until soft.
To the sautéed onions, add the San Marzano and the diced tomatoes together with all their juices.
Add salt and pepper.
Cook at a high simmer for about ten minutes.
Puree the soup by transferring to a blender in small batches or use an immersion blender directly in the pan. (If using a regular blender, return the pureed soup to the pan.)
Reduce the heat to low and whisk in the sour cream and tarragon.
Garnish each with a dollop of sour cream and a little tarragon if desired.
Notes
Any leftover soup will keep for 3 or 4 days in a closed container in the refrigerator. May be reheated in the microwave or over low heat on the stovetop.This soup also freezes well. Store in a freezer-safe container for up to 3 months. Thaw overnight in the refrigerator before reheating.