Heat 1 teaspoon oil in a large skillet over medium heat. Add the diced onion and a pinch of salt and cook, stirring occasionally, until softened and translucent, about 5–6 minutes.

Add the minced garlic and cook 30–45 seconds until fragrant, taking care not to brown the garlic.

Crumble the 1 lb sausage into the skillet and increase heat to medium-high. Break up the meat with a spatula and cook until no longer pink, about 6–8 minutes. Spoon off and discard any excess fat, leaving a tablespoon for flavor if desired.

Stir in 1 tablespoon Dijon mustard, 1 teaspoon Worcestershire sauce, 1 teaspoon salt, and 1 teaspoon freshly ground black pepper. Cook 1–2 minutes to combine and reduce slightly. Remove skillet from heat and stir in 2 tablespoons chopped fresh parsley. Transfer the filling to a bowl to cool slightly (10 minutes).

Onion Compote (extra for topping)

While the sausage cooks or in a separate small skillet, heat 1 tablespoon unsalted butter over medium-low heat. Add the thinly sliced large onion with 1 teaspoon brown sugar and 1/4 teaspoon salt. Cook slowly, stirring occasionally, until the onion is deeply softened and beginning to caramelize, about 15–20 minutes.

When onions are golden, stir in 1 teaspoon balsamic vinegar and 1/4 teaspoon black pepper. Cook 1 more minute, then remove from heat and set aside. Reserve about half for mixing into the sausage (optional) and half for topping the finished pastries.

Pastry & Assembly

Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper. Lightly flour a work surface and roll out the single puff pastry sheet to smooth any creases and make a roughly 12 x 10-inch rectangle if needed.

Cut the pastry into 6 equal rectangles (approximately 4 x 3.5 inches each) for individual pastries, or cut into 3 larger rectangles for hand-pies. If making larger pastries, adjust baking time as noted below.

If you reserved half of the onion compote for mixing, stir about 2 tablespoons of the compote into the cooled sausage mixture now to add sweetness and moisture. Add 2 tablespoons grated Parmesan to the filling if using and mix to combine.

Place about 2–3 tablespoons of the sausage filling on one half of each rectangle, leaving a 1/2-inch border around the edges. Spoon a small dollop (about 1 teaspoon) of the reserved caramelized onions on top of the filling if desired.

Lightly brush the pastry edges with a little water or egg wash. Fold the empty half over to enclose the filling and press the edges together. Crimp the edges with a fork to seal and vent the top with a couple of small slits to let steam escape.

Beat 1 large egg with 1 teaspoon cold water to make egg wash. Brush the tops and edges of each pastry generously with egg wash. Sprinkle each pastry with a little grated Parmesan if using.

Place pastries on the prepared baking sheet, spacing at least 1 inch apart. Bake in the preheated oven for 18–22 minutes, until the pastry is puffed and deep golden brown. For larger hand-pies, bake 25–30 minutes.

Remove from oven and let cool on the baking sheet for 5 minutes, then transfer to a rack. Serve warm or at room temperature with extra caramelized onions on the side.