Peel and cut the potatoes into 8 pieces. Microwave them in a heatproof container covered with plastic wrap for 5–6 minutes at 600W until soft. Blend the potatoes with water until smooth.

Cut the mentaiko lengthwise and scrape out the roe into a bowl with 100ml of milk.

Prepare the asparagus by peeling tough parts, cutting lengthwise, and slicing into 3–4 pieces.

Chop the onion roughly and sauté in olive oil with salt and pepper until softened.