Put a large pan of salted water on to boil. Scrub the potatoes and chop them into bite-sized chunks. When the water is boiling, add the potatoes. Simmer for 20 mins till the potatoes are tender.
While the potatoes simmer, peel and crush the garlic. Halve the chilli, scoop out the seeds and white pith (or leave them in for more heat) and finely chop it. Pop the garlic and chilli in a bowl. Add 1 tsp each garam masala and turmeric and a pinch of chilli powder (it's spicy, so use as much or as little as you like).
Add ½ tbsp oil to the bowl of spices and stir together. Add the minute steaks and turn to coat them in the spices. Set aside for 5 mins to marinate.
Peel and finely slice the onion. Halve the pepper, scoop out the seeds and white pith and slice the pepper into fingers. Chop the tomatoes into around 8 wedges each.
Slice the steaks into thin strips. Put a wok or deep frying pan over a high heat. Warm for 2 mins, then add the beef strips and stir fry for 3-4 mins to brown them. Scoop them out and pop onto a plate.
Add the onion and green pepper to the pan. Stir fry for 5 mins till the veg start to soften and pick up a little colour.
The potatoes should be cooked by now, so drain them well and add them to the wok along with the tomatoes. Stir fry for 5 mins till all the veg has picked up a little colour and the tomatoes have started to break down.
Add the beef back into the wok with any juices from the plate. Toss everything together for 3-4 mins to make sure it's all piping hot. Fork in the spinach so it starts to wilt.
Divide the beef chilli fry between a couple of warm bowls and serve straight away.