Peel and thinly slice the cucumber then place it in a colander.
Mix cucumber with salt and press under a heavy weight for one hour to drain.
Squeeze remaining liquid from the cucumber and transfer to a bowl.
Whisk sugar, vinegar, and pepper together and toss with the cucumber slices.
Heat stock, crème fraîche, and mustard then whisk in 50g butter and lemon juice.
Drizzle monkfish with olive oil and place into a hot frying pan.
Add a knob of butter to the pan and fry until fish is thoroughly cooked.
Squeeze lemon juice over the cooked monkfish fillets.
Serve the fish on warm plates alongside the prepared cucumber and sauce.