Place 100g of the butter into a pan over a medium heat and leave for 8 to 10 minutes, or until it melts and separates.
Strain the yellow (clarified) liquid into a separate bowl and leave to cool, discarding the leftover milky-coloured liquid.
Carefully melt the remaining 150g of butter in a pan over a low heat, then allow to cool slightly.
Remove any tough stalks from the dill, then finely chop most of it. Scrape it into a bowl, then add the shrimp and crab.
Finely grate half the lemon zest into the bowl, then squeeze in half the juice. Grate in a little nutmeg, pour in the melted butter and season with sea salt and white pepper.
Spoon into a serving bowl (or 6 to 8 smaller dishes), smoothing the surface. Top with the clarified butter, pick and scatter over the rest of the dill, then pop into the fridge for 2 hours, or until set.
Serve the potted shrimp and crab with crunchy radishes and some good crusty bread on the side, if you like.