In a saucepan, melt two tablespoons of butter, sauté celery and onions in it. Add potatoes to the saucepan. Add water to the saucepan and allow the contents to simmer till the potatoes become tender.
Melt the remaining two tablespoons of butter and mix the flour in it to make a roux. Add the roux and milk to the potatoes. Cook the potatoes mixture over medium heat till it becomes thick. Keep stirring constantly.
To this mixture, add seasonings, salmon, and the tomatoes.