To thicken this recipe some flour is added to the onion and briefly cooked before adding to the slow cooker. Tossing the flour in with the onions before adding to the slow cooker will make sure there no raw taste left with the flour. Coating the onions with the flour first will also make sure that it is distributed evenly and you don't end up with clumps of flour in the sauce.
If you are in a rush and plan to skip the pre-frying of the sausages and the onion then you can mix the flour with an equal amount of water and whisk until you get a smooth consistency. Then mix this into the liquid in the slow cooker and then cook as normal.