Put the onions in a large pan, cover with salted water and boil for about 20 minutes or until tender.
Drain and retain 300ml (1/2 pt) cooking water.
When cool carefully remove the onion centers leaving about 1/2" thick onion shells.
Chop half the centers.
Mix together the sausage meat, dry stuffing mix, ketchup, parsley and chopped onion.
Press into the centre of each onion shell.
Stand in the Slo cooker.
Add the stock cube to the onion liquid.
Bring to boil and pour over onions.
Top each with a knob of butter.
Cook for recommended time.