Preheat the oven to 350°F (180°C). Line 2 rimmed baking sheets with parchment paper that sticks up on each side by about 1 inch (2.5cm).
Heat the oil in a skillet over medium heat.
Add the onions, and cook for 5 minutes, until translucent.
Add the garlic, and sauté for 1 minute more, stirring constantly. Leave the onions to cool to lukewarm.
Mix together the cooled onions, ground pork, sausage meat, breadcrumbs, sage, nutmeg, salt, and pepper. Divide the mixture evenly into 4 parts (pat the mixture evenly into the bowl, and cut it into 4 wedges with a butter knife). Set aside.
make the pastry crust:
Stir the flax seeds into the water and set aside to gel for 10 minutes.
Measure the flour and salt into a large bowl. Cut the lard into chunks and add it to the flour.
Cut the lard into the flour with a pastry cutter or two butter knives until it is in pea-sized chunks.
Add the yogurt and the flax gel to the flour and stir with a fork to partially combine everything, then use your fingers to rub the flour into the butter. Use a light touch and mix the dough until the flour is barely all mixed in. Move the dough to a work surface and knead it 3 to 4 times to form a ball.
Cut the ball of dough into 4 pieces, then roll each piece into a cylinder about 6 inches (15cm) long.
Working with one piece of dough at a time, roll it on a floured work surface into a rectangle that is 5½ inches (14cm) wide and 12 inches (30cm) long. Use the ruler to gently push against the edges to straighten them or cut off wider pieces and patch them onto narrower sections of the rectangle.
make the sausage rolls:
Scoop up one section of the prepared sausage meat filling, and form it into a log down the center of the rolled-out dough rectangle (the long way).
Beat the egg in a small bowl with a fork, then brush some of the beaten egg down one long side of the rectangle.
Bring up the other long side of the dough up and over the sausage log (use the ruler to help lift and support the dough as you bring it over) and roll the log up so the dough overlaps with the egg-brushed edge. Gently squeeze and shape the log into an even roll, patting together any cracks that have formed.
Use the ruler to help lift the sausage roll onto one of the prepared baking sheets. Brush the roll with beaten egg and sprinkle with sesame seeds (if using).
Repeat with the other 3 pieces of dough and sausage meat, putting two logs onto each baking sheet.
Gently cut each log into 8 sections - it's okay if you don't cut perfectly all the way through - this step just helps make it easier to cut the sections after baking, and helps the heat get between the pieces. You will have 32 sausage roll bites.If you wish, you can make larger sausage rolls by cutting each log into 3 or 4 pieces.
Bake the sausage rolls for 35 to 40 minutes, until golden brown. (If using regular flour, check them at 30 minutes, as they'll brown faster). Some of the fat will be melted and pooled around the sausage rolls. Leave them in the pans to cool for 10 minutes (the oil will re-absorb into the rolls, helping keep the crust crispy). Then slice through each of the cuts again to separate the rolls into pieces and put them onto a cooling rack to finish cooling off.
Sausage rolls can be served warm, cool, or chilled. They will keep in the fridge for up to a week, and can be frozen for up to 6 months.
Makes 32 mini sausage rolls (or 12-16 longer sausage rolls).
Notes
I prefer ground golden flax seeds, as they are less noticeable in the dough, however, regular brown flax seeds work just as well.You can substitute butter or shortening for the lard, but lard gives the crispiest crust (and is also traditional with pork pies or sausage rolls).You can buy loose sausage meat, or buy your favourite uncooked sausage links and remove the casing, to use the sausage meat from inside.