Chicken breast in slow cooker

Your best bet would be a dutch oven on a low to medium low heat in the oven. You could use a regular pot in the oven, but you'd need to stir it regularly (maybe every hour) to stop everything from sticking to the sides and burning.

With regard to your comment, "what can you cook in a slow cooker", I cook almost everything in the slow cooker!

With regard to onions, I suggest that you precook the onions so that they obtain the texture that you want and then add them to the slow cooker. For example, when I cook stew, I caramelise the onions before placing them in the slow cooker, for the cooking temperature is too low for Maillard reactions.