Using your cutting knife and cutting board, be sure to dice your leek and carrots, Then mince your garlic. Next, melt your butter in your stovetop pan.
Add in your diced leek, carrots, and minced garlic. Cook on low heat until soft. Should take about 15 minutes.
Add in your heavy cream and sprinkle in a dash of diced dill, salt & pepper. Mix together and let the mixture simmer for a couple minutes.
After a few minutes, remove the stovetop pan from heat and add in your shredded cheese. Give it a quick stir, and then set aside to cool.
Preheat your oven to 375℉. Line your baking sheet with parchment paper and set aside.
Grab your puff pastry dough and roll it out on a floured surface. You want to make a rectangle shape out of the dough, long ways horizontal. Sprinkle some diced dill weed around the dough.
Next, place your salmon fillet in the center of the dough rectangle, with the length of the fish placed horizontally.
Using your wooden spatula from your stovetop mixture, scoop the stovetop vegetable content onto the salmon fillet until the entire fillet is covered.
Grab the sides of the puff pastry rectangle and fold it over the salmon fillet and vegetables. I suggest started with the long sides first, and then fold the shorter sides inwards, like you're wrapping a gift. Pinch the corners.
In a small mixing bowl, combine your egg with 1 tbsp of water and mix with a fork. This is your egg wash.
Brush the pastry with the egg wash (can use a food brush or a spoon). Then flip the pastry over and lightly score the dough with your knife. Add more egg wash on this side of the pastry and add a final garnish of chopped dill.
Place on parchment-lined baking sheet and bake in the oven for 25-30 minutes.