Heat the apple juice to a boil with the finely chopped onion and mustard seeds, and add the Teriyaki sauce. Try the marinade with salt, if necessary, add salt.
Cool the marinade to room temperature.
Put the pork steaks in the cooled marinade and leave to marinate. Marinating time is at least 20 minutes, maximum-12 hours.
Remove the steaks from the marinade and brush each steak with butter. Cook on the grill for 5-8 minutes on each side until fully cooked.
Remove the pork from the grill, cover with foil and let sit in a warm place for about 10 minutes.
Cut the apples into 0.5 cm thick slices. Also brush with oil and cook on the grill on both sides until the characteristic stripes appear.
Serve the finished pork steak with grilled apples.