Ingredients

1 tbsp sunflower oil

1 tsp unsalted butter, softened, plus

more to butter the bread

1 tsp mustard seeds

1 red onion, half finely diced, half thinly sliced

300g (10½oz) new potatoes, peeled and cut into large bite-size pieces

big pinch of salt, plus ½ tsp

1 small bunch of coriander

(cilantro), leaves picked and roughly chopped

1 tbsp grated (shredded) fresh ginger

juice of 1 lemon

1 clove of garlic, thinly sliced

1–2 green chillies, finely sliced (remove the seeds if you want less heat)

8 slices of white bread

200g (7oz) cheddar, gloucester or gouda cheese, thinly sliced

½ cucumber, peeled and thinly sliced

4 tomatoes, thinly sliced

2 tbsp chaat masala, plus optional more to serve