Cut the chicken into bite-sized pieces. Slice the red bell pepper and julienne the carrot. Chop the onion, mince the garlic, and grate the ginger.
Heat 1 tbsp oil in a large pot or Dutch oven over medium heat.
Add the chicken pieces and cook for 4-5 minutes until browned. Remove from the pot and set aside.
In the same pot, add a little more oil if needed. Add the chopped onion, minced garlic, and grated ginger. Sauté for 2-3 minutes until fragrant and the onions are soft.
Add 3 tbsp Thai red curry paste to the pot. Stir and cook for 1-2 minutes to release the flavor.
Pour in the coconut milk and chicken broth (or water). Stir well to combine with the curry paste.
Add fish sauce, brown sugar, and salt. Stir and bring the soup to a gentle simmer.
Add the bell peppers, carrots, and spinach (if using). Simmer for 5-7 minutes, or until the vegetables are tender.
Add the rice noodles to the pot and let them cook for 4-5 minutes (or according to package instructions).
Return the cooked chicken to the pot. Let it simmer for an additional 5 minutes to ensure the chicken is cooked through.
Turn off the heat and add fresh lime juice. Taste and adjust the seasoning, adding more salt or fish sauce if needed.
Serve hot, garnished with chopped cilantro, fresh Thai basil, lime wedges, or red chili flakes. Pair it with rice, spring rolls, or a fresh salad.