For the cream, clean the celeriac, cut into large chunks and cook them in the stock with the cream, then puree to a smooth paste with a blender with a little of the cooking liquid and the black garlic. Season with salt and pepper if necessary.
For the sweet-and-sour marinade, stir all the ingredients together, heat slightly and strain.
Clean the pumpkin, cut into thin slices and marinate in the sweet-and-sour mixture. Shape into rolls.
With a small cutter, cut rings out of the sourdough bread, fry until crisp in a little oil and allow to drain on kitchen paper.
Mix the olive oil with the lemongrass puree and put to one side.
Clean the scallops, fry briefly on both sides in hot oil and season with salt and pepper. Arrange the fried scallops in their shells, garnish with tiny swirls of the puree, a roll of pumpkin, the cress and the lemongrass oil.