Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius). Line two baking sheets with Gefen Parchment Paper.

Form 12 small circles of brown sugar, one teaspoon each, on each prepared baking sheet. Place small pats of butter, 1/4 teaspoon each, on top of the patches of sugar and top each one with an onion slice.

Roll out the puff pastry dough and cut out 24 circles, ensuring that the dough is wider than the onion slices by 1/2 inch (one centimeter) all around. Place a circle of dough on top of each onion and brush with egg wash.

Bake for 20–25 minutes, or until the dough puffs up and begins to turn golden. Remove from the oven and turn the tartlets over. Sprinkle with goat cheese and rosemary and/or thyme.

Bake for another five minutes, or until the cheese begins to melt. Remove from oven and sprinkle with salt to taste. Serve immediately.