Layer the cucumber in a colander with the salt, weigh down with a plate and leave for 2 hours. Rinse and dry the cucumber gently. Mix with the vinegar and caster sugar.
Melt 10g (just under half) of the butter in a small saucepan and sauté the shallot until soft but not coloured. Add the cream and bring to the boil; stir in the mustard, seasoning and lemon juice and pull off the heat. The cream will thicken immediately you add the lemon juice, so don’t add too much. If you overdo it and the sauce thickens too much, just add a little water. Add the dill and taste to see if you need to adjust anything.
Heat the oil and the remaining butter in a frying pan. Season the salmon all over and cook over a medium-high heat, skin side down first, for 1½-2 minutes or until the skin is golden and almost crispy. Turn the fillets over and cook for another 1½-2 minutes on the other side. Cook for about 30 seconds on each of the other two sides.
Turn the heat down, cover the pan and leave the salmon until cooked through but still moist – it will cook in the steam created by the moisture in the pan. How long this takes depends on the thickness and weight of your salmon fillets, but we're talking a matter of minutes. You can check by poking the tip of a sharp knife into the thickest bit of flesh – the flesh shouldn’t look glassy.
Add the dill to the cucumber and toss everything together then reheat the sauce. Serve the salmon with the sauce and the cucumber.