Preheat oven to 350 F. Coarsely chop the spring onions. Cook the macaroni noodles following the instruction on the package
Melt butter in a saucepan, add the flour and stir milk in all at once. Use a wire whisk and whisk vigorously to make sure they are blending smoothly. Bring to a boil and lower the heat to let it simmer for 10 minutes. Add in the spring onions for the last 2 minutes. Season with salt, pepper, and nutmeg. Remove from the heat. Add the beaten egg yolks into the sauce while stirring at the same time to make sure they blend in
Add the fish cubes and cooked macaroni noodles into the sauce. Mix gently to make sure all are covered with the sauce. Pour into a 8-9 inch-baking or casserole dish. Bake for 40 minutes or until the top is golden brown