Source: https://www.recetasgratis.net/receta-de-pollo-cordon-bleu-10042.html
Remove excess fat and tendons from the chicken. Make a cut along the thickest part of each breast fillet with a sharp knife. Open the fillets, season with salt and freshly ground black pepper.
Put a slice of cheese and another of ham on a part of each steak. Cover it with the other part to cover the filling.
Carefully roll the fillets with flour and shake to remove the excess. Pass them through the beaten egg and then through the bread crumbs. Put the battered fillets on a baking sheet lined with aluminum foil. Store them in the refrigerator for 30 minutes.
Heat the oil in a medium saucepan with a heavy bottom. Add the chicken and fry in two batches over medium heat for 4 minutes on each side, or until it is well done and browned. If necessary, add more oil between each batch. Serve the chicken immediately.