Step 1

Preheat oven to 425°F.

Step 2

Heat the oil over medium heat in a large skillet. Add the onion and bell pepper and sauté until soft and translucent, about 4 minutes.

Step 3

Add the ground beef and break up with a wooden spatula. Cook, stirring occasionally, until no longer pink. Tilt the skillet and use a large spoon to remove most of the rendered grease (it's okay to leave some in the skillet). Discard safely.

Step 4

Add the beef broth, tomato sauce, chili sauce, Worcestershire sauce, mustard, vinegar, brown sugar, salt, and pepper. Bring to a simmer, stirring often.

Step 5

Continue simmering over medium heat until the mixture has thickened and most of the liquid has cooked down, about 15 minutes.

Step 6

Transfer the mixture to a baking dish. Top with the cheese, and then arrange the puff pastry squares over the cheese.

Step 7

In a small bowl, mix together the beaten egg and milk (or water). Brush the egg wash over the tops of the puff pastry. Sprinkle the sesame seeds on top.

Step 8

Bake until the puff pastry is browned (but not burned) and the sides are bubbly, about 12 to 15 minutes. Remove from oven and let rest for 5 minutes.

Step 9

Serve at once, garnished with chopped scallions (if desired).