1. Fill and boil your kettle. Peel the onion and the carrot and finely chop both. Peel the garlic cloves and finely slice them (set one of the chopped cloves aside for later). Pour 1 tsp oil into a medium pan and warm to a medium heat. Slide in the onion, carrot, 1 garlic clove and the thyme sprigs. Season with salt and pepper and fry for 6-8 mins, stirring occasionally, till slightly softened.
2. Pour 500ml boiling water into a jug. Crumble in the stock cube and stir to dissolve. Thinly slice the mushrooms. Roughly chop the sorrel leaves. Trim any woody stalks out of the spring greens and shred the leaves.
3. Tip the lentils into the pan. Top up with around 400ml of the stock (save the rest for later in the recipe). Bring to the boil, then cover the pan with a lid. Reduce to a simmer and cook for 25 mins, till the lentils are tender. Stir now and then and if the pan looks too dry, turn the heat down and add a splash of water.
4. When the lentils have about 10 mins remaining, warm a large frying pan over a high heat till smoking hot. Rub the minute steaks with a pinch of salt and pepper. When the pan is hot, add 1 tsp olive oil and the steaks. Cook for 1-2 mins on each side, then the steaks out onto a warm plate and loosely cover with foil. Set aside to rest.
5. Add 1 tbsp olive oil to the frying pan and lower the heat to medium. Add the mushrooms and season with salt and pepper. Fry for 4-5 mins, stirring, till the mushrooms look golden and juicy.
6. While the mushrooms cook, stir the spring greens into the pan with the lentils. Cook for 3-4 mins, without the lid on, to wilt the leaves. Add a splash more water if the pan starts to look a little dry.
7. Stir the remaining garlic and the sorrel into the mushrooms and cook for 1 min. Pour in half the pot of double cream and the rest of the stock. Bring to a simmer and cook for 1-2 mins, till the sauce is thickened and glossy.
8. Divide the lentils between a couple of plates and top with the steaks. Spoon over the creamy mushroom sauce to serve.