Preheat the oven to 180°C, 350°F, Gas Mark 5.
To make the cookies, either rub the butter into the flour by hand or pulse in a food processor until it resembles breadcrumbs. Add the yolk and poppy seeds and mix until it starts to come together into a ball. Roll out onto a lightly floured surface to about 2cm in thickness and cut out circles with a cookie cutter.
Lay on a lined baking sheet and bake for about 10 minutes or until pale and cooked. Set aside on a wire rack to cool.
To make the soup, sweat the onion and garlic in the butter in a large pan until golden and soft. Add the squash and cook for five minutes, then add the thyme and stock. Simmer for about 20 minutes or until the squash is tender.
Allow to cool a little, remove the thyme stems then blitz in a food processor or with a hand-held blender until smooth. Season and stir in the cream. Set aside.
To make the salad, finely slice briefly blanched mushrooms and toss with the onion. Pour over the oil, lemon and add the thyme and some seasoning and leave to infuse for 15 minutes. To serve, reheat the soup without boiling and pour into bowls. Serve the cookies and salad alongside.