Season the cubed chicken breast with salt and pepper. Prepare the onion, garlic, ginger, and other ingredients.
Heat the vegetable oil in a large skillet over medium heat. Add the onion and sauté for about 3-4 minutes until soft and translucent. Add the ginger and garlic, continuing to sauté for another 2 minutes until aromatic.
Add the chicken and curry paste to the skillet. Cook for 2-3 minutes, stirring to ensure the chicken is well-coated and starts to cook. Pour in the coconut milk, fish sauce, and sugar. Stir well and bring to a simmer. Let the curry simmer uncovered for about 10 minutes or until the chicken is fully cooked through.
Add the spinach to the pot, stirring until it wilts. Squeeze in the lime juice, then taste and adjust the seasoning with additional salt, pepper, or fish sauce as needed.
Stir in the chopped cilantro right before serving. Serve the curry hot over fluffy white rice, with naan bread and garnished with sliced red chili peppers and lime wedges on the side.