Place the steak in the freezer for around 30 minutes (this makes it easier to slice thinly).
Add the butter and olive oil to the instant pot and set to saute. When its hot add the onion and pepper.
Fry for a couple of minutes and then add the oregano, garlic and season to taste.
Thinly slice the steak and add to the instant pot. Stir well to combine and roughly brown the meat. Put the lid on. Set to manual high pressure for 9 minutes.
Leave under pressure for 9 more minutes before performing a quick release.
Remove the lid and add the mustard and cheese. put the lid back on for a couple of minutes until the cheese has melted.
Serve straight away in the rolls.
Calories: 689kcal | Carbohydrates: 36g | Protein: 45g | Fat: 41g | Saturated Fat: 19g | Cholesterol: 137mg | Sodium: 622mg | Potassium: 481mg | Fiber: 2g | Sugar: 6g | Vitamin A: 473IU | Vitamin C: 2mg | Calcium: 266mg | Iron: 14mg