Heat the oven to 180°C/Fan 160°C/Gas 4. Scrub the sweet potatoes and slice them into wedges. Spread out on a baking tray. Drizzle with 1 tbsp oil and some salt and pepper. Bake for 25-30 mins, turning once, till golden and cooked through.
Meanwhile, peel and crush or finely chop the garlic. Scrape into a non-metallic bowl. Add the tamari, 2 tbsp vinegar and ½ tsp each of the oregano and ground cumin. Whisk together.
Slice the sirloin minute steaks into thin strips. Add to the tamari marinade. Turn to coat in the marinade and set aside to marinate for 10 mins.
Peel and thinly slice the red onion. Roughly chop the tomatoes. Halve, deseed and slice the chilli.
Warm a deep frying pan or wok for 1 min. Add 1 tbsp oil. Lift the beef from the marinade (keep the marinade for later) using a slotted spoon and stir-fry over a high heat for 2-3 mins till browned.
Add the onion, tomatoes and chilli to the pan. Stir fry for 2-3 mins till the onion has softened a little.
Pour in the reserved marinade and warm for 2-3 mins till it bubbles up. Add the spinach and stir till just wilted. Spoon the stir-fry into warm bowls and serve with the sweet potato wedges.