Remove puff pastry from freeze and let thaw. Preheat oven to 375.
Thinly slice white onion. Add olive oil to a large pan and then add onion along with a few sprigs of thyme.
Cook down for about 30 min, stirring every so often so the onions don't burn. Onions should get very brown, reduce and be very sweet and caramelized.
Grate Hay There Truffle Gouda and slice tomatoes.
Line a baking sheet with parchment paper. Roll out pastry and cut 12 squares. Add a tsp of caramelized onion, 2-3 tomatoes and 1 tsp of Gouda. Fold to opposite corners of each square to form little boats. Brush each pastry with the egg yolk, and sprinkle with thyme.
Refrigerate for 10-15minutes before baking in the oven for 15 minutes, or until cheese is melted and pastries are puffed and golden. Enjoy!