Pour the olive oil into your crock pot. Add the onion, tomatoes, sugar and salt. Cover and cook on high for about 3 hours. The time is variable. When you are ready to serve, melt the butter and stir in the flour before adding it to the crock pot. Then pour in the evaporated milk and puree with a stick blender. That's it. I also served with some hot noodles and rye bread. I added about a teaspoon of dried basil and used no salt. You can also add a bit more of sugar to taste.