First, preheat the oven to 375℉.
Now, slice the onions and place in a baking dish. Not a sheet pan because it's too thin on the bottom.
Place the sliced onions in the baking dish and drizzle oil over then salt and black pepper. Be sure to mix the onions around in the oil and seasoning.
Now, place into the oven for 45 minutes.
Then, at 45 minutes remove from oven and move the onions around. Place back into the oven for one hour. If the onions look like their drying out add a little more oil.
At this point the onions will be caramelized and brown. Not all of them will do this but to try and make all the onions caramelized you will burn the majority of them.
Quick tip, when onions are finished add in a tablespoon of butter and let it melt. Then, mix the butter into the baked onions before serving.