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Melt butter in a saucepan and add in the chopped leeks. Cook for a few minutes till leeks become soft. Set aside. Place the three cheeses into a bowl and add in the cooked leeks. Mix till leeks are well blended and cheese is smooth. Add in a little salt and fresh ground pepper. Set aside. To make the dough mix the flour, salt, baking soda and baking powder to a large bowl till well combined. Make a well in the center and add the sour cream. Mix with a wooden spoon till just mixed. Now, switch to your hand and mix the dough, till it comes together. Pour out onto a lightly floured surface and begin to knead for 5 minutes. This will help the dough to become smooth. Once the dough becomes smooth allow to rest for 5 minutes. Sprinkle a little flour onto a flat surface, roll out the dough to a 14 inch circle. Now, add the cheese mixture into the center of the circle. Spread the cheese mixture out to a 7 inch circle. Fold over one side of the dough over the cheese mixture. Use a pastry brush to remove any flour from the top of the dough. Fold over the next side and brush off the flour. Continue with the other sides till you create a square. Place the bread onto a cookie sheet lined with a piece of parchment paper that has been sprinkled with cornmeal. Mix the egg wash together and brush the top and sides of the dough. Place into a preheated 375 degree oven and bake for 35 minutes. Remove and allow to cool for 5 minutes on a wire rack. Serve warm.
Note, you could do this recipe a day ahead, cover with plastic wrap and refrigerate.