MANGO CHUTNEY

• 3 ripe mangoes (around 1kg) • 2 tbsp vegetable oil • 40g fresh root ginger, finely chopped • 2 garlic cloves, finely chopped • 1 red chilli, finely sliced (deseeded, if liked) • ½ tbsp nigella seeds • 2 tsp garam masala • 350g granulated sugar • 180ml white wine vinegar • 1 lemon, juice • ½ tsp freshly ground black pepper • ½ tsp fine sea salt