Cut the steak into roughly 2 inch/5cm dice.
Cut the potatoes into dice smaller than 1 inch/2.5cm and the pepper and onion into a similar size.
Pre-cook the potatoes either boiling or in the microwave until just cooked (boiled around 5min or 2min in microwave in a dish with some water).
Mix together the oil, garlic, paprika, thyme, salt, pepper and lemon juice.
Toss the steak, potatoes, onion and pepper in the oil mixture.
Cut foil squares and divide the steak and vegetable mixture evenly between them.
Seal up the foil packs and either cook on the grill around 10 min or in oven at 425F/220C around 20 min.
Open up the pack and finish the steak either directly on grill or the whole pack under the broiler for a couple minutes to brown.
Serve directly in the foil pack.