Pop the porcini mushrooms in a heatproof bowl. Pour in 150ml boiling water to cover them. Set aside for 10 mins to soak.
Finely slice the mushrooms. Peel and finely chop the onion. Trim the stick of celery. Finely slice it into crescents. Peel and grate or crush the garlic. Finely chop the parsley stalks (keep the leaves for later).
Warm 2 tsp olive oil in a pan. Add the onion, celery and mushrooms. Season with salt and pepper. Stir and cook over a medium heat for 8 mins till the veg are soft and juicy. Stir in the garlic and parsley stalks. Cook for 1-2 mins.
Stir in the porcini mushrooms and their soaking liquid. Drain the lentils. Rinse them and add to the pan. Pour in 400ml boiling water. Pop on the lid. Bring to the boil.
When the lentils and mushrooms are boiling, turn the heat down to low. Simmer for 8-10 mins so everything heats through and the flavours mingle.
While the lentils cook, finely chop the parsley leaves, chervil sprigs and chives. Stir them into the yogurt with a pinch of salt and pepper.
Taste the lentils and mushrooms. Adjust the seasoning if you think they need it. Ladle into bowls and serve with the summer herb yogurt.