I do onions a lot and really like them. I do whole onions (white and sweet yellow) and they take about an hour and a half to 2 hours at 225. This is the best way to do onions if you want them for slicing. They keep a very long time in the fridge as well. It's very simple. I usually peel off the first real layer of the onion and then just throw it on and let it go.
I have also done a halfed onion. I dig out the center pit a bit and add a Tsp of butter and a half cube of either chicken or beef boullion. Then, wrap in foil. These take an hour to an hour and a half depending on how tender you want them. You can add some mushrooms or such as well. Remember to peel off the first main layer as well.
If you want to got the short and quick route with just slices, I would slice more than you need and place them on the grate. I would think a sliced onion would need no more than 30 minutes. So, slice more than you need and start taste testing after 15 minutes and see how things shape up.