While slow cookers are incredibly convenient, they do have a reputation for blandness. This is because there’s no opportunity for the browning reactions that normally happen in the oven resulting in less big complex meaty flavours.
My solution is to start by roasting the meat and onion in the oven for 30 minutes. I use the slow cooker insert so there’s no extra cleanup. It adds an extra step but definitely worth it for the bigger flavours.