Pastry Magic: Sift flour and salt into a bowl. Rub in the butter with your fingertips until it resembles coarse breadcrumbs. Gradually add the water, mixing until a dough forms. Avoid overworking the dough, or it will become tough. Wrap in cling film and chill for at least 30 minutes.

Sausage Symphony: While the dough chills, preheat oven to 200°C (400°F). In a pan, saute onions until softened. Let cool, then mix with sausage meat, brown sauce (if using), herbs, salt, and pepper.

Assembly & Shaping: Roll out the dough on a lightly floured surface to a rectangle about 30x20cm (12x8 inches). Spread the sausage mixture evenly over the pastry, leaving a clean border along one long edge. Brush the border with water. Tightly roll up the dough, enclosing the filling, and seal the seam by pinching the edges.

Golden Glow: Cut the roll into individual sections, around 2 inches thick. Place them on a baking tray lined with parchment paper, seam-side down. Brush with the egg yolk mixture and sprinkle with a pinch of salt for extra flavor.

Baking Brilliance: Bake for 20-25 minutes, or until golden brown and cooked through. Let cool slightly before serving.