Preheat the oven to 220°C.
Start by making the roasted rhubarb.
Chop the rhubarb into 1.5cm thick chunks and place the chunks in a roasting tray.
Sprinkle with the sugar and a drop of warm water, then place in the oven.
Caramelise the rhubarb in the oven – this will take around 10 to 15 mins.
Then remove the tray from the oven, scrape away any black bits and leave the rhubarb to cool.
Lay two sheets of baking parchment on the counter and season the fish with salt, pepper and the chopped rosemary.
Dust the fillets lightly with plain flour, then lay them on the baking parchment.
Drizzle some olive oil over the fish, then slide the mackerel into the oven.
Roast the mackerel for around 10 minutes, until they’re a nice golden colour and there’s no pink in the middle of the fish (check by slicing into it).
Remove the fish from the oven and take it out of the parchment, setting it aside to rest before serving.
Add all the ingredients for the sauce – capers, sherry vinegar, chopped rosemary, and a good glug of olive oil – to a small saucepan and warm through for 3 mins until piping hot.
Take the mackerel fillets off and lay them on a serving plate along with the roasted rhubarb.
Serving Suggestions: Sprinkle the sauce with a little chopped fresh parsley over the mackerel and serve!