Chop the potatoes into 2cm chunks. Peel and thickly slice the onion.
Set a medium pan over a medium heat for a few mins. When warm, add ½ tbsp olive oil and the potatoes and onion with a good pinch of salt. Fry for 5 mins, stirring regularly, till the onion is slightly softened and some sides of the potato chunks are golden.
While the onion and potatoes are cooking, peel and grate the garlic. Roughly chop the tomatoes.
After 5 mins, add the garlic, 1 tsp each cumin seeds and oregano and a pinch of chilli flakes to the pan (they're spicy, so use as much or as little as you like). Stir for 1 min. Add the chopped tomatoes and olives. Cook for a further 4 mins, stirring now and then.
Add 1 tbsp red wine vinegar and crumble in the stock cube. Pour in 300ml boiling water. Stir well. Pop a lid on the pan and simmer for 10 mins. Pierce a potato chunk with a sharp knife. It should be tender. If not, cook for a little longer till they are.
Preheat your grill to its highest setting. Taste the casserole and adjust the seasoning. Add more chilli flakes, to taste, if you like too.
Spoon the cooked casserole into a shallow, heatproof dish. Slide under the hot grill for 10 mins till the top is golden and lightly charred.
Meanwhile, finely chop the mint leaves. Remove the casserole from under the grill. Spoon into 2 warm bowls and top with the chopped mint leaves to serve.