Preheat your oven to 200 C/180F/Gas 4. Heat 1 tbsp oil in a frying pan and fry your beef for about 5 minutes until nicely browned. Drain off any extra fat. Then add your onion, curry powder, cumin powder and chilli powder and reduce the heat, cover and cook on low for 10 minutes. Stir through the chickpeas, peas and coriander, season and taste adjusting spices if you need to. Take off the heat.
Prepare your Brick pastry. Lay out a circle and cut off about 2cm from each side. Then cut the remaining pastry into half length ways leaving you with 2 strips about 6-7cm wide. Do the same with the rest of your Brick until you are left with 12 strips.
Cover a baking tray with tin foil and brush with oil.
Take a strip and place about 1 tbsp of filling at the bottom end and then start folding it up like a concertina to make the traditional triangular samosa shape. Place each samosa onto the covered baking tray with the loose side down. When all your samosas are made, brush them all with oil all over which helps them to hold their shape and aids with the crispiness. Sprinkle them with the mustard seeds.
Bake in the oven for 15-20 minutes, turning twice, until crispy and golden. Serve with some mango chutney.