Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the butter and oil in a large frying pan, add the onion and fry for 5 minutes or until softened. Stir in the lemon rind and rice and cook for 1 minute. Mix in the wine, stock and a little salt and pepper and bring to the boil, stirring.
Pour into the slow cooker pot. Arrange the salmon steaks in a single layer on the rice, turning on their sides, if needed, so they are just below the surface of the stock. Cover with the lid and cook on low for 1¾–2 hours or until the rice is tender and the salmon flakes into opaque pieces when pressed in the centre with a knife.
When almost ready to serve, bring a saucepan of water to the boil, add the asparagus and cook for 5 minutes or until just tender. Spoon the rice into shallow bowls and top with spoonfuls of crème fraîche, the drained asparagus and salmon steaks broken into pieces. Sprinkle with chopped chives and a little extra pepper.