Heat a saucepan on high. Add spinach and cook for 2 minutes, stirring, until it wilts. Drain on paper towel. Chop coarsely and cool.
Combine spinach and fetta in a bowl. Cut a pocket in thick side of each chicken fillet. Fill with spinach mixture. Dust chicken pieces with flour, shaking off any excess.
Melt butter in a large frying pan on medium heat. Add chicken and cook for 5 minutes each side, until browned. Add stock to pan. Stir, cover and simmer on low heat for 10 minutes. Stir in parsley and season to taste.
Serve chicken with potatoes and extra spinach and drizzle with sauce. Dusting chicken or fish with flour before frying helps it to brown well and protects the flesh from drying out by forming a light case for the meat to steam gently inside. When a liquid such as stock or wine is added, the flour helps to thicken it to make a sauce.