Heat ½ the oil in a large pan. Add the bacon lardons and cook for 4-5 minutes, until golden. Remove with a slotted spoon to a bowl, leaving all the oil in the pan. Add the onion and leeks to the pan and gently cook for 10 minutes. Stir in the thyme and drained lentils.
Meanwhile, put the flour in a shallow dish and dust each cod fillet all over. Heat the remaining oil in a non-stick frying pan and cook the cod for 3-4 minutes on each side, until cooked through.
Add the stock and double cream to the pan with the lentils, and stir in the cooked lardons. Bring to a gentle simmer and cook for 2 minutes. Stir through half the parsley.
Divide the lentil mixture among 4 plates, then top each with a piece of cod. Garnish with the remaining parsley.