Heat oven to 200C/180C fan/gas 6.
Roughly halve the pastry, but make one
half a tiny bit bigger. Roll the slightly
smaller half out on a floured surface to a
circle about 15cm in diameter. Spread over
the mustard, leaving about a 2cm gap
around the edge. Mix the mascarpone,
breadcrumbs, chopped sage and garlic
together and season generously.
Arrange a few squash slices,
overlapping, to cover the mustard.
Spread with some of the mascarpone
mixture. Repeat, making slightly smaller
circles with the squash each time, and
spreading mascarpone between each
layer until the top is just one slice of
squash and you have made a rough
dome shape on the pastry.
Thinly roll out remaining pastry. Use to
cover the squash dome, gently pressing
down on the veg to push out as much air
as you can. Press the pastry edges to
seal. At this stage, you can cover and chill
the pie overnight, or freeze for 1 month.
Defrost before continuing. Brush with
beaten egg, then poke a tiny steam hole
in the top and gently score patterns over
the surface. Dip a few sage leaves in the
remaining egg and stick on top. Transfer
to a baking sheet and bake for 35-40 mins
until a skewer poked through the steam
hole goes through the veg easily.