Grate the onion into a large mixing bowl and stir in the breadcrumbs so they soak up the moisture.
Add the beef, egg, garlic, crumbled stock cube (or salt), pepper, and jalapeños, then mix everything together with your hands until it’s fully combined.
Toss in the cheddar and parmesan and lightly crumble them through the meat mixture before giving it one final mix.
Divide the filling into two logs and place each one along the edge of a puff pastry sheet.
Roll the pastry tightly around the meat, trim about 2cm off the excess edge, and seal with egg white.
Cut each roll in half, place them seam-side down on a baking tray, brush with egg yolk, and sprinkle with sesame seeds.
Bake at 200°C/425°F (or 180°C fan) for 35 minutes, then transfer to a wire rack and bake another 15 minutes until golden and crispy.