Place a heavy-bottomed stockpot over medium heat and, when it is hot, add the butter. When the butter has melted, add the onion, garlic, celery, and curry powder and cook, covered, until vegetables are tender, about 10 to 15 minutes.
Add the tomatoes and their juices, stock, basil, and brown sugar, raise the heat to high and bring to a boil. Lower the heat to low and cook, partially covered, 1 1/2 hours.
Remove the solids and place in a blender. Add the sun-dried tomatoes and process until completely smooth. Gradually return to the cooking liquid. Serve immediately or refrigerate up to three days.
To freeze: Prepare soup as directed. Allow to cool, place in a rigid freezer container and freeze.
To serve: Thaw in fridge. Reheat on stovetop or in microwave.