Sweet potato and Spinach curry

This has become a favourite of mine for a meat free Monday. The other week I had it three days in a row and I'm still not fed up of it. It's healthy, tasty and filling. I add lentils so there is no need for rice, just add them 20 minutes or so before the end of cooking time. It's a recipe I found on line from the Co-op. The cream isn't necessary but it thickens it, alternatively add coconut milk to make it vegan. 

Ingredients

Serves 4
2 tbsp vegetable oil
2 red onions, finely sliced
4 garlic cloves, crushed
5cm ginger, finely grated
2 tbsp medium curry powder
1.2kg sweet potatoes, peeled and cut into 3cm pieces
400g can chopped tomatoes
400ml vegetable stock, made with 1 stock cube
2 x 125g packs baby spinach
3 tbsp double cream (optional)
2 green chillies, finely sliced
Rice, to serve (optional)

Method
Heat the oil in a large nonstick saucepan or casserole dish
Fry the onion over a medium heat for 10-12 mins, until golden and sticky. Stir in the garlic, ginger and curry powder, then cook for 2 mins more
Add the sweet potato, chopped tomatoes and stock, then bring to a simmer
Cover and cook gently for 20 mins, or until the potato is tender
Stir through the spinach and cook for 5 mins more, then stir in the cream and bubble, uncovered, for another 5 mins, or until slightly thickened
Season to taste
Scatter the chilli slices over the curry and serve with rice, if you like