Preheat oven to 400°.
Unfold the puff pastry. Using a 4 inch round cookie cutter, cut out 4 circles.
Place 2 inches apart on a greased baking sheet. Bake until golden brown for about 14-16 minutes.
Shred the beef with 2 forks, and transfer it to a large saucepan.
Add tomatoes, pepper and vegetables and bring to a boil.
In a small bowl, mix the cornstarch and water until smooth, then stir into the beef mixture.
Return the mixture to a boil, making sure to stir constantly.
Cook and stir until thickened about 2 minutes.
Ladle the stew into 4 bowls, top each with a pastry round.