Place a large nonstick frying pan over the highest possible heat. Sprinkle the meat all over with salt and pepper. When the pan is smoking hot, put in the steaks and sear for 30 seconds on each side. Remove and set aside.
Lower the heat to medium and add the butter and onions. Sprinkle with salt and sugar and gently cook the onions until caramelized and soft, 20 minutes. Add the Armagnac and cook, stirring occasionally, until the onions become drier, 10 minutes longer.
Let cool for 10 minutes, and then blend to a smooth paste in a food processor. Refrigerate until cold (or freeze for speed), about 1 hour.
Preheat the oven to 400 degrees F/200C and line a baking sheet with parchment paper.
Roll the pastry between 2 sheets of parchment paper to 1/5-inch/1/2cm thick. For each piece of steak, cut 2 pastry rectangles slightly larger than the steak. Brush each piece of steak with some of the mustard on both sides, then place in the middle of a piece of pastry. Top the steak with a heaping tablespoon of the onion mixture. Brush the egg wash around the pastry edges and cover with another piece of pastry. Press the edges to seal the parcel, and then trim the excess pastry to leave a 1/2-inch/1cm border and crimp with a fork. Repeat for the remaining steaks.
Cut a little cross in the top of each parcel and brush the pastry with the egg wash. Place the parcels on the lined baking sheet and bake for 12 to 15 minutes for medium-rare.
Remove from the oven, cover with foil and let rest for 5 minutes before serving.