Set aside a few of the prettiest mushrooms to garnish the soup and roughly chop the rest. Warm 20ml of olive oil in a casserole pot and sauté the whole mushrooms over medium heat until gently browned and cooked through. Set aside until needed. Add another 20ml of olive oil and sauté the diced shallot until soft and translucent. Increase the heat to high, add in the chopped mushrooms and fry until soft. Scatter in the thyme leaves and season generously with black pepper and a little salt. Deglaze the casserole with the white wine and pour in the chicken stock. Crumble in the stock cubes and stir to dissolve. Remove the soup from the heat and use an immersion or stick blender to purée the soup. Stir through the cream and plain yoghurt and leave the soup to simmer over low heat for 10 minutes. Meanwhile, cook the tortellini according to the packet instructions. Once cooked, drain the tortellini and add to the mushroom soup, stirring the pasta through.