Slow cooker tomato and paneer curry

Ingredients

• 1 red onion, thinly sliced
• 4 fat garlic cloves, grated or crushed
• 2 red peppers, deseeded and cut into 2cm slices
• thumb-sized piece of ginger, grated, or peeled and julienned or finely chopped
• 1–2 green chillies, thinly sliced (or 1–2 teaspoons chilli flakes)
• 450g paneer, cut into 2–2.5cm dice
• 25g butter, cubed
• 1 x 600g jar of passata
• 1 tablespoon tomato purée
• 1 teaspoon caster sugar
• 1 teaspoon garam masala
• 1 teaspoon cumin seeds
• 1 teaspoon salt, plus extra to season
• 150ml single cream
• 1 lime, juiced
• 150–200g spinach
• small handful of coriander, stalks finely chopped, leaves roughly chopped
• black pepper