Heat oven to 200C/180C fan/gas 6. Finely chop the garlic, put half in a bowl for the dressing and set the rest aside for the chips. Toss the sweet potatoes with the oil and spread out on a large baking sheet. Bake for 25 mins, turning halfway through.
Put the fish on a sheet of baking parchment on a baking sheet, brush with a little oil, then grate over the lemon zest and season with black pepper. Set aside.
Boil the leek for 5 mins, then add the broccoli and cook for 5 mins more. Tip in the peas for a further 2 mins. Drain, return to the pan and blitz with a stick blender to make a thick purée. Add the mint, then blitz again.
Meanwhile, toss the garlic and paprika with the chips and return to the oven with the fish for 10 mins. Add the olive oil to the garlic, with the capers and dill and 1 tbsp water. Serve everything together with the caper dressing spooned over the fish.