Step 1

PASTRYUsing a large bowl, whisk together flour, xanthan gum, baking soda and saltUsing a box grater, grate the chilled butter into the flour. Using a fork, mix the butter with the flour, and then using your fingers, work the butter into the flour until crumbly.Add sour cream and mix. The dough will still be a bit dry. Add one tablespoon of ice water at a time, and using your hands, knead the dough until it begins to come together. If the dough is too dry, keep adding 1 tablespoon of ice water at a time while working the dough until it holds together. I used 5 tablespoons.Transfer dough onto a plastic wrap and form a compact disk. Wrap the dough tightly and place it in the refrigerator for 45 minutesFILLINGMelt the butter in a pan over medium heat. Add the onions and cook until softened.

Step 2

Add garlic and spinach and cook until the spinach is just wiltedAdd bread crumbs, cream cheese, and dill/thyme until combined and the cheese has melted.

Step 3

Remove the pan from the heat and stir in mozzarella. Set aside to coolPreheat oven to 425 FRoll out the pastry dough into a rectangle the size of the salmon allowing for 2 extra inches of the dough on the sides and 1 inch on the endsSeason salmon with salt and pepper and place in the middle of the pastry. Spread spinach mixture on top of the salmon, smoothing the top so that it does not spill over the sidesFold the edges of the pastry over the salmon and spinach, starting at the longer sides. Then fold the ends on top. Flip the pastry with the salmon over and place on a baking sheet lined with parchment paperSlice the top of the pastry with a sharp knife into a criss-cross pattern and brush the pastry with the beaten eggBake 25 - 27 minutes until the pastry is golden brown