Mix the sugar and sea salt together in a bowl and divide into four portions.
Layer two pieces of cling film and sprinkle one portion of the salt mixture in the center.
Place one trout fillet skin-side down on the salt and top with another portion of the mixture.
Wrap the fillet tightly in cling film and repeat for the second fillet.
Chill the wrapped fish in the fridge for 12 to 24 hours.
Unwrap the cured trout and rinse thoroughly under cold water then pat dry.
Light a handful of oak chips in a smoker and drizzle with water to smoulder.
Place fillets skin-side down on the grate and smoke covered for 60 to 90 minutes.
Remove the trout from the smoker and allow it to cool.
Whisk balsamic vinegar and olive oil together and season with salt and pepper.
Arrange shallots, beetroot, and feta on a platter and flake the trout over them.
Garnish with watercress and drizzle with the prepared dressing.