Preheat oven to 425 degrees F. In a large saucepan cook yellow or white onion and sweet pepper in hot butter over medium heat until tender. Stir in soup and milk. Cook and stir until bubbly. Stir in tuna and lemon juice.
Transfer mixture to an ungreased 1-1/2-quart casserole. Bake, uncovered, for 15 minutes.
Meanwhile, prepare biscuit mix according to package directions. Drop batter into 10 to 12 mounds on top of hot tuna mixture. Bake, uncovered, for 15 to 20 minutes more or until biscuits are golden and a toothpick inserted in centers of biscuits comes out clean. If desired, sprinkle with green onions.