Put the garlic, onions, carrots, lamb joint and stock into the slow cooker. Add salt and pepper.
Cook on HIGH for 10 hours or LOW for 12 hours.
When cooked, remove the meat to a tin, smother in the mint jelly and bake in the oven at 220C for about 20 - 30 minutes or until browned.
Remove the carrots and onions and set aside.
To make the gravy:
Using a spoon or a gravy seperator jug, pour off the fatty layer from the cooking liquid in the slow cooker. Discard the fatty part.
From the remaining cooking juices, remove a small amount of the liquid from the pan (say a tablespoon or two) into a mug or bowl and stir in the cornflour into it until it dissolves. Add a small amount at first and stir in, keep adding until you get a thick paste.
Add this 'slurry' back into the pan and stir. You can leave this in your slow cooker on HIGH for about 30 minutes, or if your pan is able to go on the hob, pop onto the hob to bubble for 10 minutes until thick.