In a large bowl, combine coconut milk, curry paste, fish sauce, and egg. Beat the mixture until the curry paste is dissolved and well incorporated.
Add flounder pieces and let marinate in fridge for about an hour.
In a large skillet, heat up oil under medium to high heat. Add the dried chiles and cook until browned, about 1-2 minutes. Add the bell pepper and saute until soft, about 4-5 minutes.
Add fish/coconut milk mixture to pan and bring to a boil. Simmer for about 15-20 minutes, when fish is white and mixture is heated through. Add in spinach and stir until wilted. Season with salt and pepper to taste. Serve with rice.