Line a 9-inch round baking dish (cake pan or pie plate) with parchment paper. Set aside.
In a skillet, heat a splash of cooking oil over medium heat. Add sliced onions and a sprinkling of salt. Cook, stirring, until onion are softened and just beginning to colour. Remove from heat and allow to cool slightly.
Lay your sheet of thawed puff pastry over-top and press down into the pan. Trim into a round shape, discarding excess pastry, if necessary. Using a fork, pierce the pastry on the bottom several times, to stop it from puffing too much as it bakes.
Place the puff pastry shell into the pre-heated oven to pre-cook for 5 minutes, then remove.
Spread créme fraîche over the bottom of the pastry. Scatter cooked onions evenly over-top. Top with rounds of goat cheese, arranged evenly over the top. Season with a bit more salt and freshly ground pepper. Sprinkle top with shredded Comté or Gruyere cheese.
For egg wash, if using: In a small bowl, beat the egg with 1 teaspoon of water. Brush the egg wash over the exposed sides of the pastry.
Bake in preheated 425F oven for 15-20 minutes, or until puffy, golden and bubbly.
Let stand a few minutes to set up, then lift out of the baking pan to a cutting board or serving plate. Top with baby arugula and a drizzle of honey. Slice and serve.
Notes
This tart is best enjoyed fresh. You can refrigerate and re-heat leftovers, but the puff pastry will loose some crispness.Be sure to read the notes above this Recipe Card, where I share more detailed tips, variations and substitution suggestions for this recipe!