Heat a glug of oil in a large pan over a medium heat. Fry the onion gently for 5 minutes. Add the garlic, ginger and red chilli and cook for a few minutes more.
Whilst that is cooking, grind the cumin, coriander and mustard seeds lightly in a pestle and mortar. Then add to the pan, along with the turmeric and garam masala and cook for 1 minute until fragrant.
Add the lentils along with the tomatoes, coconut milk and stock, then stir to combine. Season with salt and pepper and cook on a medium/low heat until thick - at least 15-20 minutes (stirring often so that it doesn't stick to the pan). Taste and add more chilli if desired.
Add a squeeze of lemon juice and the handfuls of spinach. Allow the spinach to wilt and then serve with your chosen accompaniment.