Preparation
The Method
First make the marinade. Peel the garlic and use a garlic crusher or grate on the fine side of a box grater and put in a shallow non-metallic dish with the soy, vinegar, Worcestershire sauce, oregano and cumin. Cut the steak into strips and stir into the marinade and mix well to combine. Cover with clingfilm and set aside at room temperature if you intend to use immediately or leave for up to 2 days in the fridge to allow the flavours to develop. Just allow it to come back up to room temperature before you cook it.
Preheat the oven to 180°C/ 350F / gas mark 4. Peel the potatoes and using a small sharp knife cut them into 2.5cm cubes. Place in a pan of boiling water and add a pinch of salt. Bring to the boil, then reduce the heat and simmer for 6-8 minutes until just tender but still holding their shape. Drain into a colander in the sink and then spread on to a baking sheet lined with parchment paper. Spray over one teaspoon of the oil, tossing the potato cubes until evenly coated. Roast on the middle shelf of the oven for 25-30 minutes until cooked through and golden brown, turning them once to ensure they cook evenly.
Using a small sharp knife, cut the onion in half and peel off the skin, then cut into thin wedges. Cut the peppers in half and remove the seeds and core, then thinly slice. If using the fresh chilli cut it in half and take out the seeds with a teaspoon, then thinly sliced into long thin strips. Cut the tomato in half and then cut into thin wedges.
Heat a wok or large non-stick frying pan with deep sides on a medium to high heat. Spray the bottom and sides with one teaspoon of the oil and then add the steak, leaving behind the marinade. Stir-fry for 2-3 minutes until just sealed and lightly browned, then tip on to a plate. Spray the pan with the remaining teaspoon of oil and stir-fry the onion wedges and red pepper until lightly charred. Add some of the reserved marinade and allow to bubble down, then tip in the chilli or chilli powder with the tomato and toss until combined. Add the steak back into the pan with the rest of the marinade and toss again until warmed through. Divide among plates and scatter over the coriander and serve at once with the roasted potato cubes.
Prepare Ahead
The steak can be marinated for up to 2 days covered with clingfilm on the bottom shelf of the fridge. Cook as described when ready to use.