Thinly slice the onion and separate the shimeji mushrooms into small clusters.

Cut the mentaiko in half, scoop out the contents from one half, and slice the other half into 1cm pieces.

Heat salad oil in a pan, sauté onion and shimeji mushrooms over medium heat until the onion softens. Add shrimp, season with salt and pepper, and sprinkle flour. Stir until the flour is fully incorporated.

Break spaghetti into smaller pieces, add to the pan along with water, and mix well. Cover and cook over low heat for the time indicated on the spaghetti package.

Add white sauce ingredients (butter, milk, consomme, pepper) to the pan and stir until the sauce thickens. Mix in the scooped mentaiko.