Make the seaweed butter. Blitz the toasted nori in a blender. Combine the nori and butter in a small bowl. Set aside at room temperature. If you only have unsalted butter, mix in ½ teaspoon of salt.
Make the celeriac puree. In a medium saucepan over medium-low heat, melt the butter and saute the shallot and garlic until soft and translucent. We’re not trying to brown them. Add the celeriac, cream, a pinch of salt, and enough water to just cover everything. Simmer until the celeriac is completely tender, about 15 minutes. Strain and reserve the liquid. Puree the vegetables until very smooth, adding just enough liquid for easy blending. We want a velvety smooth, spreadable puree, the consistency of sour cream or thereabouts. Transfer the puree back to the saucepan, cover, and set aside to keep warm.
Preheat the broiler. Position the highest rack 9 inches from the broiler.
Cook the fish. Place the cod on a sheet pan and season lightly with salt and pepper. Slather the fillets with seaweed butter. Position under the broiler until the fish is just cooked through (likely 4-6 minutes, depending on the thickness of the fish and the strength of the broiler). The fish is done when it’s opaque and flakes apart.
Plate. Spread a large spoonful of the puree on a plate. Top with the fish and a drizzle of the melted seaweed butter. Serve with lemon wedges and crusty bread to mop up all of that good sauce. Lightly dressed greens make a nice side.