Heat the coconut oil in a large skillet over medium-high heat. Add the chicken pieces and cook until lightly browned on all sides, about 4-5 minutes. Transfer the chicken to the slow cooker.
In the same skillet, add the diced onion, minced garlic, and grated ginger. Sauté for 2-3 minutes until fragrant and the onions are translucent.
Add the Thai red curry paste to the skillet and cook, stirring constantly, for 1 minute to release its aroma.
Transfer the onion, garlic, ginger, and curry paste mixture to the slow cooker with the chicken.
Add the sliced red bell pepper, carrot, and zucchini to the slow cooker.
Pour in the chicken broth and coconut milk. Stir in the fish sauce and brown sugar until combined.
Cover and cook on low for 4 hours, or on high for 2 hours, until the chicken is tender and the vegetables are cooked through.
About 20 minutes before serving, add the dried rice noodles to the slow cooker. Stir gently to submerge the noodles in the liquid. Cover and cook until the noodles are tender, about 15-20 minutes.
Stir in the fresh baby spinach and cook for an additional 2 minutes until wilted.
Remove the slow cooker lid and stir in the fresh cilantro leaves and lime juice.
Ladle the soup into bowls and garnish with sliced green onions, fresh red chili slices if using, and lime wedges on the side.
Serve immediately and enjoy your slow cooker Thai red curry noodle soup.