1) Preheat oven to 450 degrees (f). In a medium-size heavy saucepan, melt the margarine over moderate heat; add the onion, garlic, celery, corn, pimentos, and black pepper, and cook, covered, for 5 minutes or until the onion is soft.
2) Blend in the flour, then the chicken broth and milk, and cook - stirring constantly - until the mixture has thickened, about 3 minutes. Add the cheese, mustard, and lemon juice. Remove from heat; fold in the tuna.
3) Transfer all to an ungreased shallow 5- or 6-cup casserole. Arrange the biscuits on top, sprinkle wiith paprika, and bake, uncovered, about 10 minutes or until the biscuits are golden.