Photograph: Tara Fisher
Preheat the oven to 180°C, fan 160°C, gas 4. Cut the pastry in half, then roll into two 15cm x 30cm rectangles on a lightly floured surface. (See Jo Wheatley's rough puff pastry recipe here.)
Brush a line of harissa down the centre of each sheet of pastry, then scatter the roasted peppers over the top.
Divide the sausagemeat equally in half and roll each into a sausage shape. Place along the centre of each pastry sheet. Mix the beaten egg and milk together and use some to brush one edge of each pastry rectangle.
Bring the edges together and press to seal. Brush with the remaining beaten egg and milk mixture, sprinkle with sesame seeds and slice each into 6 or 12 even-sized pieces – depending on how big you want them to be.
Place, seam-side down, on baking trays lined with baking paper and bake for 30 minutes, or 35 minutes if you've made the larger size, until golden. Transfer to a wire rack to cool slightly. Eat warm.
Get ahead: you can make the sausage rolls up to the end of step 4 up to a day ahead. Cover and chill. They can also be frozen; defrost overnight in the fridge before baking. If you don't want the spiciness of harissa, substitute it with sundried tomato paste.