Halve the leek lengthwise and rinse under running water to remove any hidden dirt. Thinly slice the leek, discarding any dark green stem. Thinly slice the spring onion, keeping the green and white parts separate. Drain and rinse the corn. Crumble the stock cubes into a large heatproof jug, add 1L (4 cups) boiling water and stir to dissolve.
Step 2 — Cook leek
Bring a medium saucepan of water to the boil for the noodles. Heat 2 tsp oil in a separate medium saucepan over medium-high heat. Cook the leek, stirring, for 4-5 mins until softened. Add the corn and cook, stirring, for 1 min or until heated through. Transfer the mixture to a large bowl and cover to keep warm. Wipe the pan clean.
Step 3 — Cook plant-based mince
PLANT-BASED MINCE OPTION Heat 2 tsp oil in the same pan over high heat. Cook the plant-based mince, breaking up the lumps with a spoon, for 4-5 mins until browned. Stir in the sriracha sauce, then cook, stirring, for 1 min to develop the flavour.
Step 4 — Combine mixtures
Add the mince mixture to the leek mixture in the bowl, stir to combine, then cover to keep warm.
Step 5 — Make broth
Put the stock, white part of the spring onion, 2 tsp soy sauce and 1 tsp sugar in the same pan and bring to the boil. Reduce the heat to low, add the miso and sesame oil and cook, stirring, for 30 secs or until the miso is dissolved.
Step 6 — Cook noodles and serve
Meanwhile, cook the noodles in the pan of boiling water for 3 mins or until tender. Drain, then rinse under hot water to prevent the noodles from sticking. Divide the noodles and mince and leek mixture among bowls, then ladle over the broth. Scatter with the sesame seeds, remaining spring onion and a pinch of shichimi togarashi* to serve.