Holiday Brisket Pot Pie

Name: Holiday Brisket Pot Pie
Ingredients: 1 pound cooked brisket (purchased from a deli); 1/2 stick (4 tablespoons) unsalted butter; 1 cup diced sweet onions, such as Vidalia; 3/4 cup all-purpose flour; 2 cups chicken broth; 1 teaspoon red pepper flakes, optional; 1/2 teaspoon ground black pepper; 1 cup peas (canned or frozen); 1 cup diced carrots; 1 cup diced Yukon Gold potatoes, parboiled; 1 package puff pastry dough, cut into 2 large pieces to cover the top of the ramekins; 2 large egg yolks, whisked