Wash, halve, and deseed the peppers. Bake on a baking tray under a hot grill for 10 minutes until the skin blackens and starts to bubble.
Cover the peppers with a damp tea towel and stand for 10 minutes.
Meanwhile, wash and slice the spring onion.
Peel the grilled peppers, cut into strips and mix with the spring onions.
Season the steaks with salt and pepper and spread with mustard. Halve the steaks horizontally and cover with the pepper-onion mixture.
Fold the steaks over to enclose the filling and secure with cocktail sticks. Brush the steaks with oil and cook on a foil lined grill pan under a hot grill for 8-10 minutes on each side.