Preheat the oven to 200 degrees Celsius. Place the salmon fillet skin side down on a baking tray lined with non-stick paper/baking paper. Bake for 10-12 minutes, until the salmon is just cooked.
For the sauce:
In a small saucepan over high heat, put the onion together with the vinegar and the wine or orange liqueur. Let them simmer until the mixture has almost evaporated and the onions remain with about a spoonful of liquid. Up to this stage you can make the sauce in advance. To finish the sauce, return the pot to a low heat. Add the cream and bring to a boil. Add 2 cubes of butter, stirring constantly with a whisk. Once the butter is soft and smooth (not melted and separated though) add another 2 cubes of butter. Stir and continue with more butter until you have a thin mayonnaise-like sauce. Remove from the heat and after seasoning with salt and pepper, add the chives. Cover the sauce and keep it in a warm place in the kitchen until ready to serve. Alternatively, you can put it in monotic heat, which is particularly convenient.
To serve, transfer the salmon to a serving platter and the sauce to a saucer for your guests to add as much as they like.