Finely chop the onion and garlic. Roughly chop the pepper and aubergine into small pieces.
Heat a Tbsp of oil in a large frying pan, then gently sauté the onion until soft. Add the garlic, aubergine and red pepper. Put the lid on and gently simmer for about 5 to 10 minutes. Add the spices, tomato purée and harissa paste. Season with salt and pepper, stir for a few seconds, then pour in the chopped tomatoes. Using the tomato tin, add some water, just enough to cover the vegetables. Add the sugar if used.
Put the lid on and gently cook for 20-25 minutes until you get a thick sauce.
Meanwhile, set the oven temperature to 190C degrees, and bake the meatballs for 15 to 20 minutes – check the cooking time on the packaging. Alternatively, you could fry them in a pan.
When the tomato sauce is ready, add the spinach and, using a liquidiser or a handheld blender, whiz it into a smooth, silky sauce.
Put it back on the cooker, add the drained lentils and meatballs. Warm through then serve with chopped coriander scattered on top.