Season the chicken breasts with salt and pepper. Cut a slit in each breast, starting at one side, being careful not to cut all the way through.
Wash the spinach leaves, place them in a saucepan and heat until the leaves are wilted. Drain and squeeze out all the excess liquid.
Finely chop the spinach and place it in a mixing bowl. Add the feta cheese and piquanté peppers and mix well.
Divide the mixture into 4 equal parts and spoon 1 part into each chicken breast pocket.
Secure the breasts with string and sprinkle with a little soy sauce and black pepper.
Sear the breasts on both sides in a griddle pan on high heat until browned and then lower the heat and cook the chicken until just done.
Serve with boiled baby potatoes.