I'm sorry, I should have said - yes, it's puff pastry, although you can use a regular egg pastry dough for it also.
My only caveat about using the spinach instead of the asparagus, is that it acts to absorb some of the moisture coming from the salmon on the bottom crust as it bakes. The spinach has to be abolustely squeezed dry before placing it on the bottom crust. I'm not sure asparagus would serve the same function.
I have, though, seen en croute made by first plaing a very thin crepe on the bottom of the pastry, then layering the ingredients over that.