To make the sukiyaki sauce, combine all ingredients in a large skillet and bring up to a simmer. Stir and cook gently for 2-3 minutes, until sugar has fully dissolved. 

To make sukiyaki with cod, add 3 cups of dashi broth to the skillet of sukiyaki sauce. (If making dashi from instant broth granules, combine 2 teaspoons dashi granules with 3 cups water for the perfect broth flavor.) Bring the dashi and sukiyaki mixture up to a simmer. 

Add fresh cod fillets and cover the skillet. Allow the broth to simmer gently, poaching the fish. Cook for 6-8 minutes, then add the fresh spinach, wilting the leaves in the final minute of cooking. 

To serve, add cooked and rinsed soba noodles in bottom of bowl. Top the buckwheat noodles with wilted spinach, then a cod fillet. Pour over broth.

Garnish with scallions and chilis.