In a Dutch Oven on top of the stove (medium heat), or in a large sauce pan, cook the onions in the butter for about 20 minutes until nearly transculent.
Add the flour to absorb the liquid, but do not let the onions brown - about three minutes.
Add the canned tomatoes (and all of their juices!), chicken broth, sugar, salt, celery seed, and pepper. Bring to a gentle boil, then reduce to simmer and cook for about 20-25 minutes. Stirring from time to time to remove any tomatoes that may become adhered to the bottom of the pot. (uncovered)
Add the half and half. Cook just long enough to reheat after the soup was momentarily cooled with the addition of the half and half.
Remove from heat (or turn off the gas) and using an immersion blender, blend the soup until smooth or to your preferred consistency. (If you do not have an immersion blend, simply us a stand up blender, adding small batches to the blender, pureeing and then repeating until done.)
Serve with a drop of half and half and enjoy. :) (I enjoyed mine with a Croque Madame :))