Melt butter in a large pot over medium heat. Cook and stir onion and curry powder in hot butter until onion is tender, about 5 minutes. Add flour, sugar, salt, and ginger; stir until completely incorporated.
Gradually pour milk and chicken broth into onion mixture, stirring constantly; cook and stir until mixture is hot and thickened. about 5 minutes.
Stir shrimp into milk mixture and cook until shrimp are heated through, 3 to 5 minutes. Stir in lemon juice.
Spread 2/3 cup cooked rice into a ring on a dinner plate. Spoon about 1 cup shrimp curry mixture into the center of the rice ring. Repeat with remaining rice and shrimp curry. Add condiments as desired.
