Wrap the coriander seeds and peppercorns in a muslin cloth and tie them with string. Put this in a pot with about 8 cups (1 litre) of water and bring this to a boil. Add the salt, lemon zest and crushed garlic. Once boiling, add the potatoes and cook for 11 – 13 minutes until knife tender. Drain and set aside to cool slightly.
While the potatoes are cooking add all the potato dressing ingredients together in a medium/large bowl and whisk to combine.
Slice the fennel and radish and set aside in another bowl. Heat the 2 tablespoons of olive oil in a small pan and fry the mustard seeds until they start to pop vigorously. Immediately drain the oil in a fine sieve and set the seeds aside to cool. Check seasoning and add a little salt if needed.
When the potatoes are cooled (they can be a little warm), slice them in half and toss lightly in the dressing in the bowl to coat. If they are a bit bigger, cut them into 3 thick slices. Keep them aside until you are ready to plate.
Dress the fennel salad just before you plate up and serve, it can wilt quite quickly. Add the mustard oil, mustard seeds, lemon juice and zest and toss to coat.
Serve on a large plate or platter with the potatoes on the bottom and the fennel salad piled on top.