Cucumber and Salmon Salad with Mustard and Fennel

Put the vinegar, sugar, salt, pepper and mustard in a small saucepan and bring to a boil. Add the onion and simmer for 1 minute. Tip into a large bowl and chill until cold.  

Slice the salmon thinly against the grain and add to the chilled marinated onions, tossing to coat the salmon in the liquid.

Shave the cucumber with a vegetable peeler into long thin slices, discarding the inner core. Add to the salmon along with the shaved fennel and half of the roughly chopped fennel fronds. 

Transfer the salad to bowls and garnish with more fennel fronds and a drizzle of olive oil. Season with salt and pepper.