You'll surely need to do a bit of preparation in a frying pan first? (Unless you buy ready fried dried onions).
Finely chop 3-4 strong English onions and fry in butter ghee until virtually charred. Fry in 2-3 teaspoons of garlic and a handful of chopped green chillies.
Add to the slow cooker along with a large jar of Patak's vindaloo sauce and a tin of chopped tomatoes. A tablespoon of white pepper. IF you are lucky enough to have any, one each of a Bhut Jolokia and Aji Limony chilli, finely chopped. Allow this mixture to brew for a while, then about 10 mins before you are about to eat, add 1kg of frozen King Prawns.
Serve with brown rice for maximum effect.
(the Aji Limony supplies a delicate citrus tang)
Be sure you do not have to make a long journey next day.