Preheat oven to 350. Add sour cream and cream cheese to pan. Whip potatoes with a wire wisk until well mixed. Mix in onion salt, garlic salt and salt and pepper.
Spoon potato mixture into baking dish. Sprinkle with cheddar cheese. Bake 20 minutes.
Arrange tomato slices on top of casserole, sprinkle with chives and bake for 5 minutes longer.