Add 1/2 cup salt to 3 gallon boiling water. Place shrimp in boiling salted water; cover, return to a boil. Reduce heat; simmer 5 minutes; drain. Set aside.
Saut? onions and peppers in 1 cup butter or margarine 10 minutes or until tender Set aside.
Add flour to 1-1/2 quart butter or margarine; blend thoroughly. Cook until well browned; stirring frequently. Gradually add water to flour mixture; cook until thick and smooth, stirring constantly. Add saut? ed vegetables. Add apples, celery, curry powder, ginger, red pepper, garlic, horseradish, and 1/2 cup salt; simmer 20 minutes. Add shrimp, mushrooms, and lemon juice; simmer 2 to 3 minutes, stirring constantly.
Other types of shrimp may be used.
3 lb 5 oz dry onions A.P. will yield 3 lb chopped onions and 2 lb 7 oz fresh sweet peppers A.P. will yield 2 lb chopped peppers.
5 lb 2 oz fresh apples A.P. will yield 4 lb chopped apples and 2 lb 12 oz fresh celery will yield 2 lb chopped celery.
1 lb 2 oz lemons A.P. (4 lemons) will yield 3/4 cup juice