Saute onions in 2 tablespoons butter. Add ground beef. After beef is browned, add tomato sauce; Mix in vegetables. Add salt and pepper to taste.
Peel and slice potatoes 1/4 thick. Cook in boiling water for approx 15 minutes or until fork-tender. Whip potatoes with electric mixer; mix until moderately smooth. Don't over beat them; a few lumps are nice. Add 1/2 cup heated milk, 1/2 cup butter, and sour cream. Salt and pepper to taste. Whip until mixed. Adjust thickness by adding more milk, if desired.
Combine biscuit mix and 1 cup milk. The mix should be thinner than normal biscuit mix but not runny.
Spray a 9x9x2 inch pan, or any similar casserole dish. Layer half way up with the mashed potatoes. Next, spread a layer of mixed vegetables or niblet corn on top of potatoes. Then add a layer of the meat. Pour biscuit mix over meat. Melt 4 tablespoons of butter and drizzle over top. Bake at 350° for approx 35-45 minutes until top is golden brown.