Melt the butter in a heavy saucepan.
Blend in flour, sugar, and salt; heat until mixture bubbles.
Add pineapple juice gradually, stirring constantly.
Bring to boiling; stir and cook 3 minutes.
Stir about 3 tablespoons of the hot mixture into the beaten egg.
Immediately blend into the mixture in saucepan and cook 3 minutes, stirring constantly.
Remove from heat and stir in lemon juice.
Store in a covered jar.