Rinse and dry the fennel bulbs. Slice thinly on a mandoline and transfer to a pot, adding the grapeseed oil.
Bruise the lemongrass stalk with the back side of knife, then transfer to a tea bag along with star anise.
Add the tea bag to the pot. Place a piece of wet parchment paper over the fennel and roast at medium heat until tender and caramelised. It may stick a bit to the bottom of the pot.
Remove the pot from the heat and let stand for a few minutes. Stir the pot well so that the caramelised bits in the bottom dissolve. Return the pot to the heat. Let the fennel become tender and golden, then remove the tea bag. Blend the fennel smooth and add salt to taste. The consistency must be very thick. Transfer the puree to a piping bag.
Slice Gruyere cheese as thinly as possible, using a deli meat slicer if possible. Cut out circles of the cheese using a cutting ring about four centimetres in diameter. There should be 16 circles per dish.
Place half the slices on a parchment-lined baking pan. Pipe a dot of fennel puree on the middle of each circle of Gruyere cheese and carefully place another circle on top, so that it floats on top of the puree. Bake the raviolis at 90 degrees Celsius, until the top slice of cheese has melted over the fennel puree and touches the bottom slice.
Remove the raviolis from the oven and let them cool slightly, then turn them over and season with black pepper. Blend the remaining cheese with 100 grams of melted butter and strain. Pour off the water from the cheese fat when cooled.
Melt the remaining 100 grams of salted butter slowly without boiling. Pour into a transparent bowl, so the clarified butter can be seen clearly on top and the whey rests on the bottom. Let stand for a few minutes while it separates completely. Use a strainer to separate the clarified butter.
Fillet the turbot from the bone, remove the skin and divide the fish into eight pieces of equal size. Cook the turbot in clarified butter on a hot pan. Cook the prettiest side first so that it will face upward when serving.
Swirl a spoonful of yoghurt onto a plate and add a few drops of cheese fat. Place two pieces of turbot on the plate and arrange four raviolis on each piece of turbot.