Dice the tomatoes. This means cutting them into pieces about 1 inch (2 cm) across. Then blend the tomatoes until they are mostly smooth. You should end up with about 4 cups. If you have less, add more tomatoes.
Add the sugar, salt, spices to the tomatoes and mix them together. You need the sugar to keep the tomatoes from turning bitter when they are cooked.
Chop the opnion finely.
Put the butter in a pot and turn the heat on to medium. This is the pot we will eventually have the soup in. Don't use the big one unless you double the recipe.
When the butter has melted, add the chopped onion and stir the two together. Cook until the onions are soft. Stir the onions about every 2 minutes.
When the onions are soft, add the flour to the pot. Stir and cook until the flour is absorbed, and you have a sticy, gooey mess. This should take about 2 minutes.
Add the tomatoes to the soup a little at a time. Add about 1/4 cup (60 mL) at a time and then stir it in before adding more tomatoes. If you add the tomaties all at once, the flour-butter mixture will not dissolve and you will have gooey chunks in your finished soup. After you have added half the tomatoes in small increments, you can add the rest all at once.
Bring the mixture to a boil, stirring frequently (about once a minute).
Boil the soup for 1 minute. It should have the rough consistancy of applesauce.
Turn the heat down to medium-low. Slowly pour in the milk. If you pour it too quickly, it will not mix in well. Stir the soup enough to mix it in evenly.
Heat the soup until it is almost boiling. There should be steam but not bubbles. Stir it about once per minute.
Once the soup is almost boiling, it is done. Turn off the heat.