Peel and finely chop the onion. Trim and peel the carrot, then dice it to match the onion. Trim the dry ends of the celery and finely slice it into little crescents.
Warm a deep frying pan or medium pan over a medium heat for 1 min, then add ½ tbsp oil and the chopped veg. Season with a pinch of salt and pepper. Turn the heat down to low and gently cook for 6 mins, stirring a few times, till the veg have softened and picked up a little colour.
While the veg sweat, tip the lentils into a sieve and give them a good rinse. Add the lentils to the pan. Crumble in the stock cube. Add the bouquet garni. Pour in 200ml boiling water and stir everything to mix it together. Pop a lid on the pan, set the heat to medium-low and gently simmer for 10 mins till the lentils are warmed through and tender. When they're ready, take off the heat and leave in the pan, lid on, to keep warm.
While the lentils simmer, tip the walnut halves into a dry frying pan. Toast over a medium heat for 3-4 mins, shaking the pan, till they smell nutty.Tip onto a board and let them cool, then chop them as finely as you can and pop them in a bowl.
Peel and crush or grate the garlic. Add to the walnuts. Finely chop the parsley leaves and stalks and add them to the bowl. Finely grate in the lemon zest and squeeze in the juice from one half. Season with a little salt and pepper and stir to mix. This is your salsa verde.
Heat your grill to high. Line a grill pan or baking tray with foil and lay the plaice fillets on it, skinside up. Brush them with 1 tsp oil each and season with a little salt and pepper. Slide them under the grill and cook for 4-6 mins till the skin is crisp and a little browned and the flesh underneath flakes easily if you press it with a fork. You don't need to turn the plaice over; the fillets will cook all the way through from the top.
Taste the lentils, adjust the seasoning if you think it needs it. Warm them for 1-2 mins if they need it. Pluck out the bouquet garni. Spoon the lentils onto 2 warm plates. Top with a plaice fillet each and serve with the salsa verde and wedges of the remaining lemon for squeezing.