The sausage rolls can be assembled up to one day before baking, but if you do that don’t brush the surface with the egg wash until just before they go in the oven.
Mix the grouse meat and sausage meat together in a bowl, then set aside.
Heat a well-seasoned sauté or frying pan over a medium-high heat, then add a splash of oil. When it is hot, add the mushrooms with a pinch of salt and sauté until they are tender and have absorbed the liquid they give off. Tip them out of the pan and finely chop. When they are cool, add to the bowl with the grouse meat.
Heat a little more oil in the same pan over a high heat. Add the Parma ham and sauté for 1 minute. Add the shallots, chestnuts, quince and thyme, and season with salt and pepper. Sauté everything together, then set aside to cool.
Once the ingredients have cooled, add them to the meats and mix well by hand. Fry a small amount in a well-seasoned pan to taste and adjust the salt and pepper, if necessary.
Roll out the puff pastry on a lightly floured work surface with a lightly floured rolling pin into a 40 x 30cm rectangle, then transfer it to a floured baking sheet that will fit in your fridge. Lightly flour your hands and shape the sausage meat mixture into a long, even roll, then place along one long side of the pastry, about 1cm from the edge. Brush the edges with egg, then gently lift the remaining pastry over the sausage roll and press the edges together, using the floured tines of a fork to seal. Transfer to the fridge for at least 20 minutes before baking.
Meanwhile, preheat the oven to 200˚C Fan/220˚C/Gas Mark 7 and line a baking sheet with greaseproof paper.
Brush the long roll with the egg wash, then cut into 8 equal portions and place them on the baking sheet. Bake for 30 minutes, or until the pastry is golden brown. Serve hot or at room temperature. I particularly like these with a watercress and apple salad, and they go well with pickled red cabbage.