Line the base of a baking tray (approx. 30 cm in diameter) accurately with baking paper.
Spread butter on the baking paper, sprinkle over the sugar.
Caramelize for approx. 4 mins. in the centre of an oven preheated to 220 °C until the sugar turns a light brown colour. Remove the tin from the oven.
Topping and pastry
Scatter rosemary over the caramel, place the onion rings on top, season. Spread the sausage slices on top. Roll out the pastry, firmly prick with a fork and place loosely over the sausage. Make a rim between the topping and the edge of the tray.
Bake for approx. 25 mins. in the centre of an oven preheated to 220°C. Remove and leave to rest for approx. 2 mins. Use a spatula to loosen the tarte edge a little, carefully turn onto a platter.