Diced the white and light green parts of three large leeks. Also used some diced red onion we had in the fridge. Sautéed those in butter with salt, pepper, nutmeg, fresh thyme, and lemon zest.

Meanwhile, I laid out one sheet of thawed puff pastry and added strips from a second sheet around the edge. Blind-baked this at 375 F. for about 10-12 minutes. While this was baking, I let the leek mixture cool and mixed 1 container of Allouette crème fraîche, and 1/2 a container of their spreadable brie. Into this, I added about 5 oz. shredded gruyere, then the leek mixture.

When the half-baked puff pastry was ready, I spread the leek mixture on top of it and we sprinkled some more gruyere across the top. 

We put it back in to finish baking and get brown and bubbly. And that's all there was to it.