Heat a drizzle of oil in a large non-stick frying pan with a lid. Add the red onion and fry for 5 minutes on a medium to high heat until soft and starting to colour.
Stir in the spices, tomato puree and maple syrup. Stir over a heat for 1 minute until combined.
Add the tinned tomatoes, drained lentils, baby potatoes and stock to the pan. Season with salt and pepper. Give it a good stir. and squash the tomatoes with the back of a wooden spoon.
Bring to the boil. Simmer on a medium heat with the lid on for 20 minutes or until the potatoes are just tender enough to eat. (the timing will vary a lot depending on the potato size)
Crumble half the block of feta into the sauce with the spinach leaves. Give it a stir and simmer for another 5 minutes, stirring once until the spinach has wilted and the feta has melted into the sauce.
Serve with rice and naan and finish off by crumbling the rest of the feta over the top.
