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Peel the onions and cut them in half. Then cut them into very fine half rings, or into strips (julienne).
Put a deep small pan or pot over a medium heat. Melt a knob of butter in it.
Stew the onion pieces in the butter. Stir and sprinkle the Tagatesse over it.
Reduce the heat a little and let the onions simmer for about 10 minutes. Don't let them turn brown.
Take the sheet of puff pastry, cut 8 triangles and place them on the baking dish covered with a baking mat or a sheet of baking paper.
Curl the edge of the puff pastry triangles to create a high edge. Press these edges firmly. Using a fork, prick a few holes in the bottom of each pastry. (Not in the edge!)
In the meantime, the onions have softened. Pour in a generous dash of raspberry vinegar, and a little red wine. Stir and simmer the onions for 10 minutes on a low heat.
Preheat the oven to 180 ° C.
Break the egg and put the yolk in a bowl. Add a tiny dash of lukewarm water and beat the egg yolk.
Use a kitchen brush and coat the edges of the pastries generously with the mixture.
Place the puff pastry tarts in the hot oven at 180 ° C and bake until golden brown. Allow about 15 minutes for this.
Set aside the pastries. Keep the oven warm at 180 ° C.
Zipper the leaves of the thyme sprigs. Finely chop them with a sharp knife and add the thyme to the softly stewed onions. Taste and add some salt and pepper to taste.
Fill each puff pastry pastry with a spoon of the fresh onion jam.
Place a piece of soft goat cheese on each cake and bake the cakes in the oven at 180 ° C. This takes around 10 minutes on average.
Serve the tarts warm.