Heat oil in a large skillet over medium heat. Add the onion and ¼ teaspoon of salt. Cook the onion, stirring often, until soft (about 7 minutes). Add the curry powder and continue stirring for 1 more minute.
Stir in coconut milk, tomatoes, and tomato paste. Continue cooking the mixture, stirring occasionally, for 5 minutes or until the sauce thickens slightly.
Add the chicken, stir well, and cook for 5 to 6 minutes or until the meat is cooked through.
Add the spinach and cook, stirring occasionally until wilted, about 3 minutes.
Salt to taste and serve.