Remove skin from sausage and crumble into frying pan. Add chopped onion, and cook over medium heat until meat is no longer pink. If you are trying to cook with reduced fat, remove meat from pan, place in a colander, and rinse under cold water.
Place meat in a large pot; add stock and potatoes. Boil until potato is cooked.
Add spinach. Continue boiling until spinach is lightly cooked.
Remove soup from heat, stir in evaporated milk, and season to taste. Do not add any salt if using canned stock.