Cut the chicken breast in small cubes. Heat the coconut oil in a large saucepan and add the chicken. Cook until it is golden.
Add finely chopped red onion and cook for a couple of minutes until it becomes soft. Then add finely chopped garlic and ginger. Cook for 30 seconds and add Thai red curry paste. Throw in a stalk of lemongrass cut in half (you can slightly bruise it with the handle of a knife to release the essential oils). Add the chopped bell pepper, pour water and boil for a few minutes.
Add the coconut milk, soy sauce, and fish sauce. Taste and adjust the seasoning by adding salt and pepper.
In another saucepan, boil the rice noodles following the instructions on the package. Usually it takes around 4 minutes.
Distribute the noodles into individual bowls and pour the soup on top. Garnish with fresh cilantro and a couple of lime wedges.
Notes
If you don't have coconut oil, you can use vegetable oil as an alternative.
Garnish with sliced chili peppers if you want more heat.
You can use chicken thighs instead of chicken breast. Thighs are more juice, but they have a bit more fat.