In a 3-quart saucepan, saute the onion in the butter until the onion is translucent (about 3 minutes).
Add water, potatoes, spinach and salt. Cook until potatoes are tender (about 20 minutes). Longer cooking does not hurt, but turn down the heat to a simmer. Add milk and Worcestershire sauce. Reheat to near boiling, but do not boil. Stir in grated cheese. Serve immediately.
NOTES: A rich cream-style potato and spinach soup for a cold, winter day -- This recipe is adapted from one that the U.S. Department of Agriculture used to distribute to potato growers in the 1930s. It is rich and thick, almost a meal in itself, and is very easy to make.