Peel and finely chop the onion. Trim and finely chop the carrot and celery. Peel and grate or crush the garlic.
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.
Finely grate the zest from the lemon. Put the lemon to one side for later. Stir the garlic and zest 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 200 ml boiling water. Cover. Bring to the boil, then lower the heat. Simmer for 15 mins till the lentils are tender.
Slice the thick stalks off the spinach. Finely shred the leaves. Pick the leaves off the parsley stalks. Slice the lemon into wedges.
Heat your grill to high. Line a grill pan with foil. Brush with ½ tbsp oil. Lay the haddock on the pan. Grill for 8-10 mins till the fish is cooked through – it’ll flake easily when you press it with a fork.
Add the spinach leaves to the lentils. Cook and stir for 3-4 mins till the spinach is wilted. Taste the lentils and adjust the seasoning.
Ladle the lentils into warm shallow bowls. Top with a haddock fillet. Garnish with the parsley leaves and serve with lemon wedges for squeezing.