Cook the potatoes in a large pan of boiling salted water for 15-20 minutes or until tender. Drain and cool.
Cook the butter and onion in a large frying pan for 5 minutes or until turning translucent. Add the garlic and fry for another 5 minutes. Add the mustard seeds and coriander seeds, and fry until starting to smell fragrant. Add the dijon mustard and juice of 1 lemon and mix. Transfer to a bowl.
Tip the cooled potatoes onto a clean worksurface and use the flat side of a knife to lightly squash each one. Fry the potatoes in a little more oil in a large non-stick frying pan for 10-15 minutes or until starting to crisp up. Tip onto kitchen paper to drain.
In the meantime, make the aïoli. Put the egg yolks, garlic, lemon juice and plenty of seasoning in a small blender, and whizz until smooth. Slowly drizzle in the olive oil while still blending on a low speed, until it emulsifies, thickens and is smooth.
Put the crispy smashed potatoes into a bowl and mix in most of the onion butter and sliced radishes.
Spread the aïoli across a platter and pile the potatoes on top. Finish with a drizzle of the remaining butter and lots of fresh dill.