In a large pot, heat the vegetable oil over medium heat.

Add the chopped onion and sauté for about 3-4 minutes until translucent.

Stir in the minced garlic and grated ginger, cooking for an additional minute until fragrant.

Add the chicken pieces to the pot and cook until they are no longer pink, about 5-7 minutes.

Mix in the red curry paste, stirring to coat the chicken and onion mixture evenly.

Pour in the chicken broth and coconut milk, bringing the mixture to a gentle simmer.

Add the fish sauce, lime juice, sliced mushrooms, and bell pepper, and continue to simmer for 10 minutes.

Stir in the baby spinach until wilted, and season with salt and pepper to taste.

Ladle the soup into bowls, garnish with chopped green onions and fresh cilantro, and serve hot.