Season the raw salmon fillet, roast briefly in a little oil while turning, cool with the lid closed.
Pick apart with a fork. Mash with basil, smoked salmon, yogurt, juice of one lemon and mustard. Add buttermilk and soup or stock, season with salt and freshly ground pepper. Cover and refrigerate for a few hours.
Serve: Divide soup evenly into chilled plates or glasses, garnish with cucumber, salmon roe and basil.
Tip: Use plain or light yogurt, yes, as needed!