For the cream of butternut squash soup, preheat the oven to 200C/400F/Gas 6.
Place the squash and garlic onto a baking tray, drizzle with 2 tablespoons of the olive oil and place the rosemary on top. Roast for 30-40 minutes, or until tender. Remove from the oven and set aside one quarter of the squash mixture, a clove of garlic and a sprig of rosemary.
Heat the remaining oil in a frying pan until hot, then add the onion and fry for 1-2 minutes. Add the remaining cooked squash, garlic and rosemary. Stir in the stock, bring the mixture to the boil, then reduce the heat and simmer for 3-4 minutes.
Blend the butternut squash mixture and cream in a food processor to a purée. Set aside.
For the croutons, heat the butter in a medium frying pan until hot then add the diced bread. Cook for 2-3 minutes, stirring regularly, or until golden-brown on all sides. Set aside to drain on kitchen paper.
Meanwhile, for the chicken and squash tortellini, place the flour into a large bowl and season with a little salt and freshly ground black pepper. In another bowl, whisk together the egg yolks and 3 of the whole eggs. (Whisk the remaining egg in a separate bowl and set aside.) Pour the egg mixture into the flour and stir to combine to form a firm dough.
Add the olive oil to the dough a teaspoon at a time and mix until the dough becomes pliable (you may not need all of the oil).
Turn the dough out onto a lightly floured work surface and knead until smooth and elastic. Place into a bowl, cover with clingfilm and chill in the fridge for 30 minutes.
Blend the reserved squash mixture and the chicken breast in a food processor and chill until needed.
Using a pasta machine, roll the pasta dough out according to the machine instructions, passing it through the rollers a few times on the thickest setting, then working down to the thinnest setting.
Place the rolled sheet of pasta dough onto a floured work surface and use a 7cm/2in pastry cutter to cut out circles of dough.
Place a spoonful of the reserved chicken mixture into the centre of a pasta circle. Brush the edges of the pasta with the reserved beaten egg.
Fold the pasta circle over to form a half moon shape, pressing down gently around the filling to seal. Holding both ends of the pasta, twist slightly to form a shell-like shape.
Repeat the process with the remaining pasta and filling. Place the tortellini onto a baking sheet lined with baking paper until you're ready to cook them.
To cook the tortellini, place a few tortellini at a time into a large saucepan of salted boiling water and cook for about 1-2 minutes, or until they float back to the surface, then drain the pasta.
To serve, warm the soup and ladle into each of 4 serving bowls. Place the tortellini in the centre on top of the soup. Sprinkle the croutons on top and garnish with the fresh chervil.
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