Mackerel is one of my favourite ﬁsh—rich in healthy fats. This is an excellent ﬁsh to introduce into your diet if you haven’t already done so. Many people think of sweet crumble when they think of rhubarb, but here its acidity cuts through the oiliness of this mackerel beautifully—so don’t be put oﬀ. Give it a try, you won’t be disappointed.
Thinly slice the fennel with a mandolin and put into a bowl. Pour the cider vinegar over it and top up with water until the fennel is just covered. Cover with plastic wrap and rest in the fridge for an hour.
Preheat the oven to 400°F.
Slice the rhubarb into 2-inch pieces and place on a baking sheet. Drizzle 3 tablespoons of the orange juice and all of the maple syrup over the rhubarb and mix. Season with black pepper and roast in the oven for 15 minutes.
Drain the fennel and pat dry on a tea towel. Flake the mackerel ﬁllets and place on a plate with the watercress, rhubarb, fennel and walnuts. Whisk the olive oil with 1 tablespoon of the orange juice and drizzle over the salad. Sprinkle with the orange zest and some black pepper, and serve.