Onions peel, halve and in strips cut. Heat 20 g butter in a pan. Sauté onions for 5-7 minutes. Add sugar, honey, wine and vinegar and simmer covered for about 30 minutes.
Defrost puff pastry slices side by side at room temperature. Season onions with salt and pepper. Remove onions from the heat and let them cool down.
Place the puff pastry slices on top of each other and roll out rectangularly (approx. 20 x 48 cm). Cut the dough into 16 rectangles (approx. 5 x 12 cm). Put approx. 2 tsp. of the onion filling on each rectangle, leaving 3-4 mm of edge free.
Place the tartlettes on a baking tray lined with baking paper and bake in a preheated oven (electric cooker: 225 °C/ convection oven: 200 °C/ gas: see manufacturer) for 15-20 minutes.
Remove the skin from the sausages. Cut the sausage into slices. Heat 20 g butter in a large frying pan and fry the sausage slices for 3-4 minutes, turning them over.
Remove the puff pastry tartlets from the oven. Spread slices of sausage on the onion puree, season with pepper and serve warm.