Begin by browning the beef in a cast iron skillet over medium heat. Add diced onion and thyme, cooking until softened, about 5 minutes. Pour in the Guinness, and let it simmer until thickened.

In a large bowl, combine flour and cold butter, rubbing until it resembles coarse crumbs. Gradually add cold water until it comes together. Chill for at least 30 minutes.

Roll out the chilled dough and cut into 5-inch circles. Spoon beef mixture onto one half, fold over and crimp edges. Brush with egg wash.

Preheat oven to 350°F (175°C). Place pies on a baking sheet lined with parchment. Bake for 18-22 minutes or until golden brown.

Let the pies cool on a wire rack for a few minutes. Serve warm with mustard or salad.