In a large pan I use my pressure cooker pan put water on to boil and add stock.

Then add all tomatoes

Let them boil again and add milk the strain them leaving the pulp behind and putting back the rest of the sieved soup back in pan.

You can let this simmer a bit add salt pepper and a spoonful of marg, if you want it a bit thicker.
Then mix flour and milk cold and add to soup and it Will thicken