Method

• 1.
  Preheat the oven to 100°C.
• 2.
  For the charred onion broth, heat oil in a large saucepan over medium-high heat. Add onion and cook, stirring occasionally, for 10 minutes or until golden. Add mushroom, garlic and thyme, and cook, stirring occasionally, for 5 minutes or until mushroom starts to caramelise. Add soy, mirin and 8 cups (2L) water, and bring to the boil. Cook for 25 minutes or until liquid is reduced by half. Strain through a fine sieve into a heatproof bowl and set aside.

• 3.
  Combine egg yolks and oil in a small ovenproof bowl, transfer to oven and bake for 20 minutes or until yolks are soft set. Using a slotted spoon, carefully transfer yolks to a plate.
• 4.
  Meanwhile, heat extra oil in a frypan over high heat. Add steak, reduce heat to medium-high and cook for 3 1/2 minutes each side for rare, or until cooked to your liking. Transfer to a plate and stand for 10 minutes to rest.
• 5.
  Thinly slice steak and divide among shallow serving bowls. Divide broth among bowls and place an egg yolk “in each bowl. Scatter with red vein sorrel leaves and serve with mustard.