Sprinkle salt on cod fillets and let sit for 15 minutes. Pat dry with paper towels and cut into 4 pieces.

Wash sweet potato thoroughly, cut into half-moon slices, and soak in water for 5 minutes. Drain. Slice onion thinly and separate shimeji mushrooms into small clusters.

Heat salad oil in a pot over medium heat. Add cod and cook until lightly browned on both sides. Remove from pot.

Wipe the pot clean with paper towels. Melt butter, add onion, and sauté over medium-low heat until softened. Add sweet potato and cook for 3 minutes, then add shimeji mushrooms and sauté until softened.

Sprinkle flour over the vegetables and stir until no dry flour remains.

Add milk, water, salt, pepper, and consomme powder. Stir and simmer for 5 minutes until thickened.