In large saucepan, combine oil, pink salt and fennel slices and stir to coat. Cover and cook over medium heat, stirring occasionally, for 10 minutes, until fennel is softened. Reduce heat, if necessary, to prevent fennel from browning.

Add water, cover and bring to a boil. Reduce heat to low and simmer gently for 20 minutes, until fennel is very tender. Uncover and allow to cool for 10 minutes.

Carefully transfer soup to a blender and blend until smooth. (Alternatively, use an immersion blender, but it won’t be as smooth.) Stir in lemon zest and blend again until it’s incorporated. Keep soup warm.

Brush salmon with olive oil and arrange skin-side down in a cool, dry non-stick skillet; you may need to do this in batches. Cook undisturbed over medium heat for 8 to 10 minutes, until skin is golden and crispy. Briefly press down with a spatula to ensure fillets are making full contact with pan. Flip them over and cook for another 1 to 2 minutes.

To serve, ladle soup into 4 wide, shallow bowls. Top each with a grilled salmon fillet and garnish with hand-torn fennel fronds. Sprinkle with flake salt and serve warm.