I also use yellow onions most of the time. But there's nothing like a baked Vidalia onion. And, as for Vidalias, the flatter they are, the sweeter they are. I learned this from the Vidalia Onion Council.
One of my favorite ways to prepare them is to plan on one smallish to medium onion per person. Peel the onions, on one side make a small "well" by removing some of the onion with a melon baller. Place the onions, well side up, in a baking dish. Add water to about 1/4 inch deep to the dish. Put a beef bouillon cube in each onion's well and a pat of butter, not margarine. Cover and bake at 350 degrees for about 1 hour. Pure heaven will await. These are super served with roast beef or roast pork.