Barramundi Baked in Puff Pastry

Barramundi is a beautiful white fish found in and around Australia and the Asia Pacific, once considered quite a delicacy. I've enjoyed it in Australia but haven't ever until just now, seen it in the US. So I decided to use it for this rather elegant preparation of fish baked in a crispy puff pastry. But you could use most any fish fillet for this dish.
2 Baramundi (or other white fish) fillets
a few pads of butter
2 leeks, sliced in half horizontally, white portions
1 orange or yellow pepper, sliced
1 Tablespoon whole grain Dijon mustard
1 12-inch square of puff pastry
1 egg whisked with 1 Tbsp water

Preheat oven to 375F. Melt a pad or two of butter in a skillet and place the leeks cut side down in the better. Allow the leeks to brown slowly for about 8-10 minutes, turning once. Cover the skillet to steep the leeks as they cook. Meanwhile sauté the peppers in a bit of butter in another skillet for about 8-10 minutes, until edges brown slightly.Gently roll pastry out on a clean, floured surface to a length equal to the two fillets laid out end to end. Lay leeks down evenly in center third of the pastry. Top with the fish fillets, and paint the mustard onto the fillets. Cover the fillets with the peppers. Fold one side of the pastry over the fish and paint the egg-wash on the pastry. Fold the other side of pastry on top, making sure it overlaps the other dough side by at least 1/2 inch. Pinch the ends closed and paint all remaining dough surfaces with egg-wash. Prick a few air holes in the dough and bake for 30 minutes, until the crust is nicely browned. Slice and serve.