1. Cut the potatoes in half, season and dress with olive oil, roast for 25 minutes at 200°C. Remove from oven and add the radishes, cut in half.
2. Add the honey and juice of 1 lemon, roast for a further 15 minutes.
3. Cook the lentils until tender. Drain and cool. Dress with the zest of one lemon, seasoning, a little olive oil and roast for 15minutes at 200°C.
4. Peel the horseradish and grate finely, mix with the buttermilk and juice of half a lemon and season.
5. Mix the dijon mustard and white wine vinegar together and brush over the mackerel and sprinkle with the oats. Bake for 8 minutes at 190°C.
6. Mix the potatoes and radishes with the sorrel.
To serve: Portion into bowls, sprinkle with the crispy lentils and dress lightly with the horseradish buttermilk. Finally top with the mackerel.