In a large heavy bottomed skillet over medium heat, add the olive oil

Add the sliced onion and sprinkle with a pinch of salt

Turn the heat down to low and sauce the onions for about 45 minutes, stirring every so often. The caramelization happens low and slow so don't try to rush it. You'll know they are done when they are golden brown and silky looking. Set aside

Preheat your oven to 400°F

Unfold the puff pastry and roll it out just slightly

Place it on a sheet pan that is lined with parchment paper

Using a sharp knife or a pizza cutter, score a 1/2'' line around the perimeter of the tart creating a border that will be the "crust"

Pierce the inside box of the puff pastry all over with a fork so it won't puff up so much in the oven

Brush with pastry all over with the egg

Bake for 8 minutes. Cool. The pastry will not be completely cool yet

Take the Boursin cheese out of the fridge to soften while the pastry is cooling

When the pastry is cooled, spread a layer of cheese in the in middle leading up to the crust edge

Spread a layer of the onions on top of the cheese

Arrange the asparagus on top of the onions. You can do this is some obsessive compulsive way like I did, or you just lay them straight. Whatever works for you.

Spray the asparagus with a little oil and top with a pinch of salt. Bake for another 15-20 mins, or until your asparagus are crispy and the pastry is golden.

RAWR!