Preheat oven to 425F (not convection or fan-assisted).
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 reheat leftovers, but the puff pastry will lose some crispness.Tips!
Try to just cook the onions just until softened, as they will brown up further as they cook in the oven.
You don't absolutely need to line your baking pan with parchment, but if you have parchment, I recommend it for easy removal and cutting.
Be sure to read the notes above this Recipe Card, where I share more detailed tips, variations and substitution suggestions for this recipe!