Salmon Wellington (saumon en croûte or coulibiac) can be wrapped in phyllo dough, brioche or puff pastry. If using puff pastry though, wrap in crepes first. Make pastry sheet not thicker than 0.12 inch/3mm. Bake on baking tray lined with parchment or perforated baking mat. Use a pizza stone to ensure pastry doneness.
Blanch spinach, drained and squeeze out any remaining moisture using paper towels; chill. In a food processor, puree scallops with salt, add egg whites, cream, butter and spinach; refrigerate until ready to use.
Searing fish remains optional. Carefully, lay phyllo dough sheet onto work surface and brush with butter generously. Repeat until done. Pipe out scallop spinach mousseline slightly larger than the fish filet, and perpendicular to the sheets width. Cover with remaining mousseline, and roll into a log; seal sides using more butter. Prick a few times the Wellington using a tooth pic. Chill an hour or for up to a day prior baking.
In a small saucepan, reduce Chardonnay, vinegar, shallots, add some herbs such as thyme, a bay leaf and a few black peppercorns. Lower the heat to medium-low and whisk in cold butter (a few cubes at the time). Whisk swiftly making sure that the mixture never boil and stay around 122ºF/50ºC. Season with salt to taste. Sieve and keep just warm; whisking every so often. If it separates for some reasons, blend using an immersion blender to smooth out.
Preheat oven to 425ºF/220ºC. In a hot and large oven-proof non-stick frying pan or sautoire, drizzle some oil and sear the Wellington for about a minute on each side and pop in the oven for 26 minutes or until internal temperature reaches 118ºF/48ºC. (Reduce baking time to 22 min if the fish filet has been previously seared). Remove wellington from pan and let rest on cooling rack for 10 min before cutting.
Slice Wellington crust with a serrated knife first and swap to carving knife before going through the fish. Serve with beurre blanc and chives. Enjoy!