Place the miso paste, peanut butter, and sesame oil in a small saucepan. Add half the chicken stock and whisk until smooth and well combined. Then mix in the remaining chicken stock. Place over high heat, cover, and bring to a simmer.
Remove the lid and drop the udon noodles into the broth. Cook for about 2 minutes, using chopsticks or tongs to help loosen them up.
Turn the heat down to low so the broth is just gently simmering. Crack the egg into a small bowl, then carefully pour it into the broth. Let it poach gently for 2-3 minutes until the white is set and the yolk is still soft (or cook it to your liking).
Carefully place the baby spinach into the broth, doing your best not to disturb the egg. Cook for another 30 seconds, just until wilted.
Use tongs and a ladle to transfer everything into a bowl. Top with sliced spring onion, a sprinkle of sesame seeds, and a drizzle of chilli oil. Eat immediately while the egg is jammy and the noodles are still beautifully slurpy.
Notes
This recipe is flexible with noodles; frozen udon works as well, just add 1-2 minutes of extra cook time.