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 heat too long or shrimp will over-cook and turn to rubber.) Serve over fluffy rice with two or more accompaniments on top: chutney, shredded coconut, diced bananas, sliced avocado, pineapple chunks, bacon bits, salted almonds, Spanish peanuts or currant jelly.
My father usually makes this on Christmas Eve because he believes the dish is more flavorful the next day. This, however, may be an excuse to not have to attend midnight church service. Also, note that at some time during the meal he will ask, "I wonder who else in this community is having shrimp curry for Christmas dinner?" Tradition has all four kids rolling their eyes and responding, "Just us, Dad." Later in the meal, Pop will say, "You know, I really think this is my best batch ever." All agree heartily and shout, "Merry Christmas!"