Spinach and Onion Puff Tart - River Cottage

Ingredients

• 350-400g spinach
• 2 tbsp rapeseed or olive oil
• 1 onion, quartered and finely chopped
• 1 garlic clove, cut into slivers
• 1 tsp chopped thyme leaves
• 375g good-quality ready-made all-butter puff pastry (ready-rolled is useful)
• 50g Cheddar or other hard cheese, grated
• Freshly ground black pepper and sea salt (optional)