Shrimp Cheddar Chowder

1 cup finely sliced onions
1 Tbsp butter
1 Tbsp flour
3/4 cup water
1 cup diced potatoes
1/2 cup sliced celery
3/4 tsp salt
1/2 lb cooked shrimp
1 cup milk (or more)
1 cup sharp cheddar cheese, shredded

Cook onions in butter; blend in flour
Stir in water, celery, potatoes and seasonings
Cover, simmer 20 minutes, or until potatoes are tender
Add remaining ingredients
Stir until cheese is melted