In a saucepan, saute onion and green pepper in butter. Blend in flour until smooth. Gradually stir in milk; bring to a boil over medium heat. Cook and stir 2 minutes. Remove from heat; stir in soup, tuna, lemon juice and salt. Pour into an ungreased 13x9-in. baking dish.
Preheat oven to 400°. For dough, combine biscuit mix and milk until blended. On a lightly floured surface, roll dough into a 12x9-in. rectangle. Sprinkle with cheese, pimientos and parsley. Roll up, jelly-roll style, starting with a long side. Cut into 1-in. slices; place over tuna mixture. Beat egg and water; brush over the swirls.
Bake, uncovered, 20-25 minutes or until top is lightly browned.