Cook the potatoes in boiling water with the stalks from the basil, the stock cube and a pinch of salt. Drain the potatoes, and crush them with a fork. Mix with the butter, olives and salt and pepper to taste. Push into a ring to form a cylinder.
Pan fry the sardine with olive oil and garlic for 2 minutes, then finish under the grill for a further 2 minutes.
Whisk together the mustard and vinegar, gradually adding 2 tbsp of olive oil. Add the beetroot and basil.
Serve the sardines atop the potatoes, place the rocket on top, drizzle the relish around the plate and serve.
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