Short Rib Wellington Potpie

INGREDIENTS

• 2 1/4 lb. boneless beef short ribs, cut into 1-inch dice
• Kosher salt and freshly ground pepper, to taste
• 2 Tbs. olive oil
• 1/4 lb. prosciutto, cut into 1/4-inch squares
• 3/4 lb. cremini mushrooms, quartered
• 8 Tbs. (1 stick) unsalted butter, cut into cubes
• 1/2 cup all-purpose flour
• 1/2 cup red wine
• 1 1/2 Tbs. beef demi-glace
• 3 cups beef stock
• 1 tsp. chopped fresh thyme
• 1 bay leaf
• 1 1/2 cups pearl onions
• 1/4 cup chopped fresh flat-leaf parsley
• 1 sheet puff pastry, 10 to 11 inches square
• 1 egg, lightly beaten with 1 tsp. water