Do a bit of prep by caramelizing the onions, in a small skillet over medium-low heat, stirring regularly, about 30 minutes. You could also buy caramelized onions or onion jam.
Cook up the pancetta in a small skillet until crispy and evenly browned. Transfer to a paper towel to drain.
Preheat oven to 400 degrees F. Lightly flour your work surface. Unfold the puff pastry (mine was a rectangle, yours might be a circle!) and roll it out with a rolling pin until it’s a bit thinner in height. Transfer it to a parchment-lined baking sheet.
Using a knife, score a 1/2-inch perimeter onto the puff pastry, all the way around. Poke a bunch of holes in the center, too.
Add the creme fraiche mixture to the center and smother all around until there’s an even layer.
Add the apple slices, caramelized onions, thyme leaves, Gruyere and pancetta. Brush the edges with the egg wash.
Transfer the tart to the freezer for 5 minutes. Preheat the oven to 400 degrees F. Bake for 15 to 20 minutes, until the edges are golden brown.
While the tart is in the oven, toss the arugula with a teaspoon of olive oil and a few pinches of salt. Top the tart with the arugula once it comes out of the oven. Slice up and serve!