Soak the plank in water for at least 45 minutes (or soak ahead of time and freeze).
Rub the plank with olive oil, placing it in the oven while you preheat the oven.
Place salmon skin-side down on plank. Massage 2 Tbsp. olive oil into the salmon; sprinkle with the seasoning. Bake until firm, and flakes easily, about 15 minutes.
While salmon is baking, saute the onion in 1 Tbsp. olive oil. Add the flour and butter; stir.
Pour in the milk (or cream), salt and pepper to taste. Stir until thickened.
Add the salmon chunks (about 2 cups), beans, and defrosted peas and carrots to the sauce; gently stir.
Pour mixture into a prepared (sprayed) 9×13 pan.
Unfold the thawed puff pastry onto a lightly floured surface. Roll pastry to 11×14 size, to seal any cracks. Place on top of the salmon mixture.
Score the topping with a knife; pinch the edges and corner to seal the pie. Brush the top with egg white.
Bake for 25-30 minutes, until brown and puffy on top. Remove from the oven and allow to cool for 5 minutes before serving.