I think Ina Garten has recommended your method for roasting red onion slices. In her case, they were for a salad but she too said to save them for hamburgers, etc.
But this made me remember ATK’s onion soup technique: Start the onions in a heavy Dutch oven on the stovetop but have the bulk of the caramelization time happen in a low oven. I’ve never done it because I can’t lug a heavy pot in and out of the oven a few times. But it does seem to create a fantastic amount of deeply browned fond to deglaze in addition to properly caramelized onions that you don’t risk burning.
