Preheat oven to 400 degrees F. Roll out two-thirds of the pastry into a circle that can easily line a 10-inch loose-bottomed tart pan. Line the pan, and trim off the excess pastry. Roll out the next piece to make a thinner top; place this piece on a piece of parchment paper, or a plate. Chill the bottom and top in the freezer for 10 minutes.
Cover the tart bottom with parchment paper, and cover with baking beans. Blind bake for 15 minutes, to a golden, not overly brown color. Remove from the oven, take out parchment and baking beans, and bake for another 5 minutes. Let cool.
As the shell is baking, cook broccoli florets in a pot of boiling water. The florets should be tender but crisp, approximately 2 minutes. Rinse broccoli under cold water, and let dry.
Melt butter in a pan, and saute leeks on a low heat for 10 minutes, so they soften but don’t color. Add mustard, cream, water, chives and a few cracks of black pepper. Stir to incorporate, and remove from heat.
Assembly: Spread the leek and cream mixture over the bottom of the tart. Spread the broccoli on top. Add the Gorgonzola. Brush the rim of the tart pan with the egg, and then place the pastry top over the filling. Press and seal the tart, and cut off any excess pastry. Glaze the top with the beaten egg and bake for 30 minutes (you’re looking for a golden brown top.) Take out of the over and cool for 3-5 minutes before removing from the pan.