Cut the sweet potato into 1cm thick slices and quarter them. Slice the onion thinly. Cut the salmon into bite-sized pieces.

Place butter in a microwave-safe container, cover loosely with plastic wrap, and heat at 600W for 20 seconds. Add flour and mix well.

Heat salad oil in a frying pan over medium heat. Add sweet potato and cook until both sides are browned. Cover and steam on low heat for 4 minutes.

Add onion and salmon to the pan and sauté over medium heat until the onion softens.

Add the milk, miso, and salt and pepper. Bring to a boil.