Slice the onion thinly and trim the root of the shimeji mushrooms, separating them.

Cut the sausages diagonally into 5mm slices.

Cut the udon noodles into bite-sized pieces, approximately 2cm squares.

Heat salad oil in a frying pan over medium heat. Sauté the onion until translucent, then add shimeji mushrooms, sausages, and salt and pepper. Stir until coated with oil, then add udon and stir until evenly coated. Remove from the pan.

Wipe the pan clean with a paper towel. Add butter and melt over medium-low heat. Stir in flour and cook until no longer powdery. Gradually add milk, stirring until thickened. Season with salt and pepper. Return the sautéed mixture to the pan and mix until well combined.