2 small red chillies, halved, deseeded, finely chopped
600g fresh udon noodles
200g Swiss brown button mushrooms, thickly sliced
1 bunch baby choy sum, trimmed, cut into 6cm pieces (see note)
200g firm tofu, drained, cut into 1cm cubes
shredded green onions, to serve
Combine stock, oyster sauce, sugar and chillies in a large saucepan. Cover and bring to the boil over medium-high heat.
Meanwhile, place noodles in a large heatproof bowl. Cover with boiling water. Stand for 2 to 3 minutes or until noodles soften. Drain. Separate noodles with a fork. Divide noodles between serving bowls.
Add mushrooms to stock mixture. Cover and cook for 1 minute. Add choy sum. Cook, uncovered, for 1 to 2 minutes or until bright green. Stir in tofu. Ladle soup over noodles. Top with green onions and serve.