Heat vegetable oil in a large pot or wok over medium-high heat.
Add diced onion, minced garlic, and grated ginger. Cook for 3-4 minutes until fragrant and onion is translucent.
Stir in red curry paste and cook for 1 minute, stirring constantly to prevent burning.
Add chopped lemongrass, coconut milk, and chicken broth. Stir well to combine.
Bring mixture to a simmer, then add chicken pieces. Cook for 8-10 minutes until chicken is nearly cooked through.
Add carrots and bell peppers. Simmer for 5-7 minutes until vegetables are tender-crisp.
Stir in fresh spinach and cook until wilted, about 1-2 minutes.
Remove lemongrass pieces before serving.
Taste and adjust seasoning as needed.