Heat the oil in a 3-quart saucepan over medium heat. Add the onion and cook, stirring, until soft but not brown, about 6 minutes.

Season the chicken with salt and pepper. Add to the pan. Cover loosely and cook, turning once, for 15 minutes, or until lightly browned on both sides.

While the chicken is cooking, blend the peanut butter and chicken stock in a blender until smooth. Set aside.

In the blender, a food processor or a mortar and pestle, chop the garlic and red pepper almost to a paste.

Uncover the pan and add the peanut butter mixture and the garlic-red pepper mixture. Stir to blend and cook for 10 minutes more.

Add the sweet potatoes, 1½ teaspoons salt and ¼ teaspoon pepper and continue cooking for 15 to 30 minutes, or until the sweet potatoes and chicken are cooked.

Add the lemon juice and spinach and cook, stirring, for 2 minutes, or until the spinach is wilted.

Serve over the rice.