Place flour in a plastic bag and toss the knuckles in to coat them - do this just before cooking or the flour will seep into the meat and make it slimy.
Heat a tablespoon of olive oil in a large pot and brown meat in batches on all sides, to seal. Remove and set aside.
Add remaining oil and sauté garlic, onion and leeks for about three minutes.
Add carrots, celery, knuckles and half the parsley.
Cook for about 10 minutes anmd add lentils.
Toss to coat, pour in stock, tomatoes and sugar. Season to taste.
Bring to the boil, reduce heat and simmer very gently, partly covered for bout two hours.
Stir in the tomato paste and simmer for a further 30 minutes, until thick and velvety.
Season well and toss in the remaining parsley, a squeeze of lemon juice and top with finely sliced lemon peel.
You could use a ham hock in place of the lamb knuckle - replace lamb stock with chicken stock and you can even add a dried-up Parmesan rind for extra depth.
Shredded spinach is nice to add just before seving.