In a pan with a lid, melt the butter over medium heat and sauté the onions until they are transparent.
Add the tomatoes and rosemary and cook them until they are juicy, for about 10-12 minutes.
Turn off the heat and remove the rosemary.
With and immersion blender, process the tomato and onion mixture until it is smooth.
Add the water, cover and simmer for 20 more minutes.
Salt to taste.
To make it richer you may want to add some more butter.
Garnish with parsley.