Wash the radishes well, cut off their tops and tear up to use as the salad leaf. (If your radishes are already trimmed then regular salad leaves will do as your salad). Thinly slice all the radishes, keep to one side.
In a bowl place the vinegar, the rapeseed oil and sugar. Then season with salt and pepper and mix.
Add the radishes and mix, then add the cucumber and chives and mix well.
Preheat a barbecue or, alternatively, heat a grill or a non-stick pan if it's raining. Season the mackerel with salt, pepper and drizzle it with a little more oil. Cook for about three minutes each side, depending on the cooking method.
While the fish is cooking, add the radish leaves (or salad leaves if using) to the bowl and mix well.
To serve, share the salad between four plates and add the dressing. Place a fillet of cooked mackerel on each plate and, garnish with the borage flowers.