Cut the potatoes into bite-sized pieces, rinse in water, and drain. Place them in a microwave-safe container, cover loosely with plastic wrap, and microwave at 600W for about 5 minutes. Mash the potatoes while hot, mix in mayonnaise until smooth, add cooked hijiki, and season with salt and pepper.

In a bowl, mix the breadcrumbs and olive oil.

Divide the potato mixture into portions and place them in heatproof containers (such as ramekins). Sprinkle the breadcrumb mixture on top. Place on a baking tray and toast in a toaster oven for about 3 minutes until golden brown.