Scrub the sweet potatoes and prick them all over with a fork. Wrap loosely in kitchen paper and microwave on high for 6-8 mins till tender and cooked through. No microwave? Simply bake in the oven for 45 mins-1 hr at 200°C/Fan 180°C/Gas 6.
If you were using the microwave, heat your oven to 190°C/Fan 170°C/Gas 5. Pop the sweet potatoes on a baking tray. Brush them with ½ tbsp olive oil each. Season. Bake for 30 mins to crisp up the skin.
Grate the zest from the lime. Juice it. Finely slice the chilli, flicking out any seeds if you prefer less heat. Peel and finely grate the ginger. Combine them all in a bowl. Add the beef. Turn to mix. Leave to marinate for 10 mins.
Peel and finely slice the red onion. Shred the lettuce. Toss the lettuce with 2 tsp olive oil and some salt and pepper. Put to one side for later.
Warm ½ tbsp olive oil in a wok or deep frying pan. Add the onion. Fry over a highish heat for 5-8 mins till the onions are golden. Lift out of the pan. Drain on a plate lined with kitchen paper.
Add the beef to the pan (keep the marinade). Stir fry for 4-5 mins till the beef is browned and cooked through. Tip in the marinade. Cook and stir for 1 min to warm everything through.
Split the sweet potatoes open. Fluff up the flesh with a fork. Pile the beef and the warm marinade into the jacket potato skins. Top with the onions.
Top the sweet potatoes with spoonfuls of the yogurt and a few of the coriander leaves. Serve with the lettuce on the side.