Preheat the oven to 190C/Gas 5.
In a heavy-based pan, heat a splash of olive oil until shimmering then add the fennel. Cook over a medium heat, stirring occasionally, until soft but not coloured. Season with salt and pepper, then add the garlic to the pan. Increase the heat and continue to cook for 30 seconds or a minute until fragrant, being careful to avoid burning the garlic. Remove from the pan, strain off any excess moisture, and allow to cool.
While the fennel is cooling, remove the sausage skins and add all to a large mixing bowl. Add the breadcrumbs and season with salt and pepper. Once cooled, also add the fennel, then mix with your hands until everything is well combined.
Cut the pastry into three equal rectangles, and in the centre of each piece, add a dollop of the sausage mixture to form a long sausage running lengthwise through the middle of the pastry rectangle.
Beat the egg and lightly brush one long edge of each pastry rectangle with the egg wash, then beginning with the dry side, roll the pastry over the meat filling to form long sausage rolls.
Seal firmly, but without allowing any filling to escape, and repeat with the other pastry pieces.
Cut each long sausage roll into four smaller sausage rolls (or more, dependent on size preference) and place seam-side-down on a baking sheet lined with baking paper, leaving some space between each. Cook in batches if necessary to avoid overcrowding the baking sheet.
Brush the top of each sausage roll with the egg wash and sprinkle over a pinch of both fennel seeds and black sesame seeds.
Place in the oven and bake for 30-40 minutes, until golden brown and cooked through.
Allow to cool slightly before eating.
