Heat your oven to 200°C/Fan 180°C/Gas 6. Scrub the potatoes and chop them into small chunks, around 2cm across. Tip them into a roasting tin. Add ½ tbsp olive oil and season with salt and pepper. Roast in the oven for 30 mins.
Peel and finely chop the onion. Trim and finely chop the carrot and celery. Tip the onion, carrot and celery into a pan with 4 tbsp water. Season. Cover. Sweat over a low heat for 10 mins till the veg are softened but haven’t picked up too much colour. If the veg start to catch, turn the heat down and add a bit more water.
Peel and grate or crush the garlic. Stir the garlic into the pan. Cook and stir for 1 min till the pan smells sweet.
Drain and rinse the lentils. Add them to the pan. Pour in 200ml boiling water. Cover. Bring to the boil, then lower the heat. Simmer for 15 mins till the lentils are tender.
While the lentils cook, add the haddock to the tin with the potatoes. Roast for 10-15 mins till the haddock is white and flakes easily when pressed with a fork.
Slice the thick stalks off the spinach. Finely shred the leaves. Pick the leaves off the parsley stalks. Stir the Worcestershire sauce into the lentils. Add the spinach leaves. Cook and stir for 3-4 mins till the spinach is wilted. Taste the lentils and add more Worcestershire sauce, salt or pepper if it needs it.
Ladle the lentils into warm shallow bowls. Top with a haddock fillet and some potatoes. Garnish with the parsley leaves and serve.