1) Clean the mushrooms by wiping them with dry kitchen paper. Don't wash them! Separate the stems, trim off any bad parts, and coarsely chop the stems. Slice the mushroom caps 1/2cm thick and, if they are big, cut them into bite-sized pieces. Set aside.
2) To make the stock, heat the olive oil and 15g of the butter in a large pot. Add the chopped mushroom stems, the onion, carrot, the sprig of thyme, 1 tsp salt, and 1/2 tsp pepper and cook over medium-low heat for 10 to 15 mins, until the vegetables are soft. Add 1.4L water, bring to a boil, reduce the heat, and simmer uncovered for 30 mins. Strain, reserving the liquid. You should have about 1L of stock. If not, add some water.
3) Meanwhile, in another large pot, heat the remaining 115g of butter and add the leeks. Cook over low heat for 15 to 20 mins, until the leeks begin to brown. Add the sliced mushroom caps and cook for 10 minutes, or until they are browned and tender. Add the flour and cook for one minute. Add the white wine and stir for another minute, scraping the bottom of the pot. Add the mushroom stock, minced thyme leaves, 1 1/2 tsp salt, and 1 tsp pepper and bring to a boil. Reduce the heat and simmer for 15 mins. Add the single cream, double cream, and parsley, season with salt and pepper, to taste, and heat through but do not boil. Serve hot.