Clean the leek, peel the carrots. Wash and drain both. Cut carrots lengthwise into thin slices with a peeler. Cut the leek lengthwise into strips with a knife.
Bring a large saucepan with plenty of salted water to the boil. First add the noodles and cook for about 1 minute. Then add the vegetables and cook for another 5-6 minutes (depending on the package information).
Pour the vegetable noodles onto a sieve, rinse briefly under cold water and drain. Wash the salmon fillet, dab dry and cut into 4 slices. Heat 20 g butter in a pan and fry the salmon for 2-3 minutes, turning them over.
Season with salt and pepper. Arrange the vegetable noodles and salmon on a deep ovenproof dish. Put 100 ml of cold water into a small pot. Stir in sauce powder and bring to the boil while stirring.
Switch off the stove. Cut 50 g of cold butter into pieces and fold in. Stir in cream, frozen herbs and prawns. Heat the sauce again briefly and season to taste with salt, pepper and lime juice.
Spread the sauce over the salmon and vegetable noodles and gratinate in the preheated oven (electric cooker: 225 °C/ convection oven: 200 °C/ gas: level 4) for 10-15 minutes. Garnish with parsley.