To make sauce, add about 1 teaspoon of dashi powder to 1 cup of boiling water. Let stock simmer for about 5 minutes, and remove from heat and let cool. When the dashi stock is cooled, add milk, masago, and Japanese mayonnaise, and mix using a whisk. In a large pan, melt unsalted butter over medium heat. Add the all-purpose flour and stir until it is well combined. Add the dashi-milk-masago into the pan and stir well. Let sauce cook for about 7-8 minutes until thickened, stirring occasionally. Season with salt and sugar. Add the cooked fettuccine into the pan and combine pasta with sauce. Serve with poached eggs, seaweed, and green onions.