Fried potato masala toastie

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