Cook the sausage, separate from its oil. Put the sausage aside and put the oil in a pan. Add vegetable oil to the sausage oil to make ~1/3 cup. Turn on low heat, add flour to oil, mixing well, to make a paste. Add a lot of milk, turn heat up, stir so it doesn't burn.
When it thickens, add sausage and tomatoes (including their oil). Spice to taste.