Place the diced chicken thighs to a bowl. Add oil, curry powder, turmeric, salt, and pepper. Toss well to evenly coat. Let it sit for 10 minutes or longer if you have time. This helps the flavour develop.
Heat a large pan over medium high heat. Once hot, add the chicken in a single layer. Do not overcrowd the pan. Let it cook undisturbed for a few minutes so it develops a deep golden colour. Turn and continue cooking for 6 to 8 minutes until cooked through. Remove from the pan and set aside.
In the same pan, add a little more oil. Add the onion, garlic, and ginger. Cook for 2 to 3 minutes, stirring, until softened and fragrant. Add the red curry paste and cook for 1 to 2 minutes. This step helps bring out the flavour of the paste.
Pour in the coconut milk and chicken stock. Stir well to combine and scrape up any bits from the bottom of the pan. Add fish sauce and brown sugar. Bring to a gentle simmer and let it cook for a few minutes so the flavours come together.
Add the udon noodles straight into the broth. Use tongs or a spoon to gently loosen and separate them. Add the pak choy and let everything simmer for 2 to 3 minutes until the noodles are heated through and the greens are just tender.
Return the chicken to the pan right before serving. Let it simmer for about 1 minute to warm through while keeping the edges golden.
Taste the broth and adjust if needed. Add a squeeze of lime for balance, then serve hot with your choice of toppings.