Cook the potatoes in a pan of salted boiling water until tender, then drain.
Once cool enough to handle, slice the potatoes thinly and arrange on a large serving plate.
Remove the heads from the sardines, then slit from the head end to tail and open the sardines out, flesh-side down. Press along the body with the heel of your hand, then turn the sardines over and carefully lift out the bones.
Place the sardines onto a baking tray and season with salt and freshly ground black pepper. Grill for 3-4 minutes, or until cooked through.
Meanwhile, whisk the olive oil, mustard, lemon juice and chives together in a bowl until well combined. Season with sugar, salt and freshly ground black pepper.
To serve, arrange the sliced tomatoes on top of the potatoes, drizzle over a little olive oil and sprinkle with the oregano. Place the sardines in the centre of the circle and spoon the dressing over the whole dish.