So I would roast (and then de-seed & skin) the peppers and courgettes
Whilst that's happening I would sweat onion and garlic in a little oil then add the spices followed by the red lentils and buckwheat. I'd then add the tomatoes (i'm thinking canned) and chilli, plus the stock and simmer for about 10mins before adding the chickpeas (again assuming tinned) and roasted veg then continuing to cook. Just before the end I would season and add the lemon juice to taste.....
I might also consider blending half of the finished soup and stirring it back into the rest just so that it isn't too "bitty" and to intensify the flavour