Spicy Potato Tagine with Olives

A North-African one-pot where the olives are not a garnish but a backbone — salty, bright, and standing up to the spice. Cheap, forgiving, and better the next day.
Olives belong in cooking far more than most kitchens use them. Here they cut through the warm spice and the soft potato with a hit of salt and brightness.
What you need (serves 4)
800 g waxy potatoes (chunked); 1 onion; 3 cloves garlic; 1 tsp each cumin, coriander, sweet paprika; a pinch of chilli; a strip of preserved-lemon or fresh lemon peel; a tin of chopped tomatoes; a good handful of green olives — Gordal or Picholine, pitted; good olive oil; coriander leaf to finish.
Method
Soften the onion and garlic in olive oil; add the spices for a minute until fragrant; add potatoes, tomatoes, lemon and a splash of water. Simmer covered until the potatoes are nearly tender (~20 min), then stir in the olives and cook 10 more — adding them late keeps them firm and bright. Finish with coriander and a thread of raw oil. Serve with bread or couscous.
Add olives near the end, not the start — long cooking turns them grey and bitter. And taste before you salt: the olives bring their own. Use a meaty green olive that holds its shape, not a soft black one.