Preheat the slow cooker if necessary; see the manufacturer's instructions. Slash the trout on each side 2–3 times with a sharp knife. Mix the spices and a little salt and pepper on a plate, then dip each side of the trout in the spice mix.
Heat the oil in a frying pan, add the trout and fry until browned on both sides but not cooked all the way through. Drain and arrange in the slow cooker pot with the fish resting on their lower edges and top to tailing them so that they fit snugly in a single layer.
Add the remaining ingredients to the frying pan with any spices left on the plate and bring to the boil, stirring. Pour over the trout, then cover with the lid and cook on high for 1½–2 hours or until the fish breaks into flakes when pressed in the centre with a knife.
Lift the fish carefully out of the slow cooker pot using a fish slice and transfer to shallow dishes. Spoon the sauce over and serve with warm bread to mop up the sauce, if liked.