Preheat the oven to 400°F / 200°C. Pulse 2 slices of white bread into crumbs in a food processor. Squeeze the meat from 12 pork sausages and mix with the crumbs and 2 tablespoons of chopped sage.
Halve 500g of puff pastry. On a lightly floured counter, roll each half into a thin 8 × 18in / 20 × 45cm sheet.
Divide 3 tablespoons of caramelized onions between the sheets, spreading them in a strip along one long side and leaving a 1¼in / 3cm border. Shape half the sausage meat into a log on each strip of onions.
Brush the borders with 1 beaten egg, fold the pastry over, and press to seal. Trim, tuck the edge under, and cut each roll into five pieces.
Place on baking sheets, brush with the remaining egg, and sprinkle with 2 teaspoons of sesame seeds. Bake for 35-40 minutes, until golden and cooked through.
