Preparing the mushroom broth: Remove stems from the mushrooms. Melt butter in a large saucepan and add the onions. Cook slowly for 3-4 minutes, then add the button and Portobello mushrooms.
Cook for 5 minutes with the lemon juice, then pour in the mineral water and simmer for 30 minutes.
Sieve and lightly salt the mushroom stock and let it cool down. Then, very quickly whisk 2 egg whites. Pour the cold stock back into the saucepan and add the whisked whites, then bring it to the boil and let it simmer very slowly for 10 minutes. Drain the stock slowly and carefully through a muslin cloth to obtain a very clear stock.
Preparing the tempura: Mix all the dry ingredients together. Weigh and add the same quantity of water less 5g to obtain a creamy batter. Keep aside at room temperature.
Remove the head from the prawns, peel off the skin but leave the tail on. Remove the black vein on the back of the prawn and set aside in the fridge.
To serve, bring the broth to the boil. Put in the instant noodles (without the seasoning) and cook for 2 minutes. Remove the noodles and divide them between 8 soup dishes. Garnish with some chopped chervil.
Heat some sunflower oil to 180°C and plunge the prawns into the tempura batter, holding them by the tail. Dip them immediately into the hot oil and fry until they turn golden. Drain the cooked prawns on kitchen paper, then place two on each skewer. Bring the broth to the boil again and pour it over the noodles. Add some fresh Enoki, then place the prawns on top, garnish with more chervil leaves and serve immediately.