In the bowl of a food processor, add your chestnuts, onion and herbs. Whizz until fine (but not a paste). Place in a large mixing bowl. Add your breadcrumbs, garlic and sausagemeat. Mix well with your hands.
Cut 4 triangles from the 4 corners of your puff pastry, then cut slits down either side of your central panel. Pop your stuffing mix in the middle of this. Fold over the edges of your pastry, brushing one edge with egg wash to make sure it sticks. Create your plait, folding in the edges as you go.
Brush the entirety of your braid with egg wash.
Bake for 35 minutes at 190°C, or until nicely golden.