Can onion based recipes such as bolognese be cooked in slow cooker?
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.