Remove and discard the dark green layers of the lemongrass until you get to the lighter green part. With a knife, cut into 4-inch lengths. Using the back of the knife, pound the ends of lemongrass to release flavor.
In a pot over medium heat, heat oil. Add onions, garlic, and ginger, and cook until softened.
Add chicken and cook, stirring occasionally, until lightly browned.
Add fish sauce and continue to cook for 1 to 2 minutes.
Add coconut milk, chili peppers, and lemongrass. Bring to a simmer.
Cover, lower heat, and cook for about 20 to 25 minutes or until chicken is cooked through.
Add papaya and cook for about 4 to 5 minutes or until papaya is tender and sauce is thickened.
Remove lemongrass and discard. Season with salt to taste.
Add spinach and cook until just wilted. Serve hot.