Slow-roasted Tropea onion, onsen tamago with dulse paper

  • 2
  • 5 hours plus time for the onions to marinate overnight
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This recipe from Koya's Shuko Oda does not use many ingredients, but makes the most of each of them. She first roasts the sweet, red Tropea onion for four hours, before marinading it overnight in a concentrated dashi made with Mizkan Honteri, a mirin-style seasoning which makes a great low alcohol replacement for mirin. Then the caramelised onion is topped with a mixture of deep fried and dehydrated dulse seaweed, as well as a quiveringly soft onsen tamago.

First published in 2023




Slow-roasted tropea onion

  • 4 Tropea onions, leave the tops on if they are in season
  • 2 tbsp of rapeseed oil
  • salt



  • 100g of fresh salted dulse seaweed
  • 200g of rapeseed oil, plus extra for brushing

Onsen tamago



Preheat the oven to 120°C/gas mark 1


Rub the onions with the oil, and season with salt. Roast in the oven for 3 hours 30 minutes to 4 hours

  • 4 Tropea onions, leave the tops on if they are in season
  • 2 tbsp of rapeseed oil
  • salt

While the onions roast, make the concentrated dashi. First, soak the kombu in the water for 2 hours

  • 15g of kombu
  • 800ml of water

Add the soaked kombu and soaking liquid to a pan, and then gently bring it to a simmer. Once it reaches a simmer, add the bonito flakes and turn off the heat

  • 15g of bonito flakes

Strain the dashi through fine mesh sieve or cloth into another pot. Add the Mizkan Honteri, usukuchi soy sauce and rice vinegar. Bring to the boil again, and then set aside


Once the onions are roasted, cut them in half lengthwise, and then transfer them to a dish. Pour over the dashi and let them marinade in the fridge overnight


To prepare the seaweed, first wash the dulse in cold water several times to remove most of the salt. Pat dry as well as you can with a tea towel

  • 100g of fresh salted dulse seaweed

Line an oven tray with foil, and with oil. Add half of the dulse, and dehydrate it at 50°C until crispy - around 20 minutes


Once crispy, leave the seaweed to cool then blitz it in a coffee grinder or powerful blender to a coarse powder, about the texture of coarsely ground black pepper


The next day, make the onsen tamago. Bring 1 litre of water to a rolling boil, then turn the heat off. Add 200ml of cold water and lower the eggs gently into the water. Place a lid on the pan and leave the eggs in the water for 12-13 minutes. Once the time is up, carefully transfer the eggs to a bowl of cold water and leave until completely chilled 


Add the vegetable oil to a deep pan and heat to 200°C

  • 200g of rapeseed oil

Fry the other half of dulse until it is crispy, transparent and bubbles begin to appear on the surface. Before adding each piece of dulse, be sure to make sure it is unfolded and fully spread out. And be careful - the oil will spit!


To serve, place the onion in a shallow bowl with a little puddle of dashi in the bottom. Place the onsen tamago in the middle and scatter the fried dulse on top along with the dehydrated dulse powder

First published in 2023

Shuko Oda spent much of her life living between Japan and the UK, making her the perfect person to combine traditional Japanese cooking and seasonal British ingredients at Koya – one of Soho’s most cherished restaurants.

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