Melt butter in a double boiler. Add onion. Simmer until tender. Stir in flour and other dry ingredients. Gradually stir in chicken broth and milk. Cook over boiling water, stirring until thickened. (Could be 10 minutes). Add shrimp and lemon juice. Heat to serving temperature. (Be sure not to head too long or shrimp will over-cook and turn to rubber.)
Serve over fluffy rice with 2 or more accompaniments on top: chutney, shredded coconut, diced bananas, sliced avocado, pineapple chunks, bacon bits, salted almonds, Spanish peanuts, or currant jelly.