Sauté the garlic and onion with the olive oil until softened. Dice the perch and chop the prawns, add to the onion and toss together. Season well with salt and pepper and cook for 2 minutes before blending to a paste. Mix in the ricotta and add the chopped dill using a piping bag, fill the cooked pasta shells with the prawn and perch stuffing. Next, melt the butter in a pan and mix in the flour and form a roux, mixing constantly. Make 500ml of fish stock using the stock pot and boiling water and whisk in the milk powder. Gradually pour into the roux, whisking continuously on low heat. To serve, warm up the filled pasta shells in the oven, drizzle with the veloute and sprinkled with parmesan.