Spicy Tuna Melt with Pickled Chillies

For the pickled chillies

100g chillies

200g caster sugar

200ml apple cider vinegar

200ml water

For the tuna melt

4 slices thick-cut sourdough bread

120g Tuna drained

1 tbsp chilli oil

1 tbsp crème fraîche

2 tbsp mayonnaise

1 tsp Dijon mustard

1 small red onion, finely diced

1 lemon, zested

3 tbsp unsalted butter (at room temperature)

1/2 tbsp golden rapeseed oil

100g cheese