1. Cut butter into the flour and salt with a food processor or pastry blender until mixture resembles a coarse meal. Small pieces of butter should still be visible in the mix.
2. Stir in the sour cream to form a dough. Form dough into a rough rectangles about an inch thick, wrap it in plastic and rest in the fridge for 20 minutes.
3. Roll the dough out into a rectangle about 2 to 3 inches longer than the salmon fillet you are using and a bit more than twice the width of the piece of fish. (Alternatively you can roll the dough into 4 smaller rectangles and prepare the servings individually.) If necessary, fold the thinnest tail section of the fish under the fillet to make it approximately the same thickness throughout.
4. Place salmon on one half the pastry.
5. Season the salmon with salt and pepper and a squeeze of lemon. Sprinkle the salmon with dill or tarragon.
6. Fold the dough over the salmon and pinch the edges together to seal. Brush the top of the pastry with an egg wash of one beaten egg and 1 tbsp water. Cut a couple of small slits in the top of the pastry with the tip of a sharp knife. Chill for at least ½ hour before baking in a preheated 375 degree F oven for about a half hour to 40 minutes depending upon the thickness of the salmon until the pastry is a uniform golden brown.

To prepare the Dijon Cream Sauce

1. In a small saucepan over medium heat, saute the olive oil and garlic for only a minute to soften but not brown the garlic. Add the wine.
2. Simmer until volume of wine is is reduced by half, then add the whipping cream, Dijon mustard, chopped tarragon or dill and a pinch of salt and pepper.
3. Simmer very slowly until sauce thickens enough to coat a metal spoon. Serve with the salmon.