Preheat oven to 425 degrees. Lightly butter 11/2 to 2-quart casserole and set aside.
Stir-fry onion in butter in a large heavy skillet over moderate heat, until lightly browned, about 5 to 8 minutes. Blend in flour and salt, then broth. Add milk and heat, stirring constantly until thickened and smooth, 3 to 5 minutes. Mix in tuna, sour-cream, pimiento and lemon juice; set off heat, but keep warm.
Cheese Swirl Biscuits:
Sift flour, baking powder, and salt together into a large mixing bowl. Add shortening and cut in with pastry blender until texture of coarse meal. Add milk and fork briskly to make soft dough. On a lightly floured surface, roll the dough into a 12 by 8 by 1/4-inch rectangle. Scatter cheese evenly on top, then jelly-roll style, starting from short end, to form roll 8 inches long. With floured sharp knife, slice 1-inch thick. Spoon tuna mixture into casserole. Space biscuits evenly on top, arranging so spiral designs show. Bake, uncovered, until biscuits puff and brown slightly -- about 20 minutes. In a saucepan with 1/2 cup boiling water add frozen peas, follow directions on the back of package. Drain. In a glass bowl, toss peas, with butter and dried mint.