Pre-heat oven to 180c/350f, and lightly grease a pie tin and set aside (see my notes for sizes and alternatives), and remove pastry sheets from the freezer to defrost.
Heat the olive oil (2 tsp) in a heavy-based pot over medium-low heat. Add roughly chopped brown onion (1 medium) and fry for 3 minutes or until the onion starts to soften.
Increase the heat to medium-high, then add lamb mince (500g/1.1lbs) and continue to fry until lamb mince is well browned and most of the moisture has evaporated and there is some nice brown colour in the base of the pot (see my notes).
Add roughly chopped bacon (2 rashers) and sliced button mushrooms (250g/9oz) to the lamb mixture in the pot and continue to fry for 2-3 minutes, until the mushrooms start to soften. Then add crushed garlic (3 cloves) and finely chopped fresh rosemary (1 tbsp) and fry for another minute.
Mix in plain flour (1½ tbsp) and fry for 1 minute, then add tomato paste (2 tbsp) to the pot, frying for another minute.
Add dry red wine (½ cup) and bring to a simmer. Allow to simmer for 1-2 minutes or until wine has reduced slightly.
Add beef stock (¾ cup), and can of tinned cherry tomatoes in juice (400g/14oz), crushing the cherry tomatoes directly in the pot with a potato masher to break them down. Bring to a simmer, and simmer over very low heat for 15 minutes then allow to cool for about 10 minutes while you line the pie tin. Add fine salt (¾ tsp - or to taste) to lamb mixture and mix well.
Line the base of your pie tin with the sheet of shortcrust pastry, trimming and patching to fit if needed.
Poor the cooled lamb mixture into the pie tin, then place the sheet of puff pastry on top of the filling and trim to fit, pressing the edges to seal.
Brush the top of the pie with the beaten egg (if using), then cut a small slit in the centre of the pie so that steam can escape, and bake for 35 to 40 minutes, or until the pastry is puffed and golden.
Allow to sit for 5-10 minutes before slicing.