Heat oven to 350° F. In a large bowl, combine the zucchini, mushrooms, tomato, potatoes, and beef and set aside. (See Roast Beef with Horseradish Potatoes recipe.)
In a small bowl, whisk together 1/4 cup warm water and the flour. In a large saucepan, bring the chicken broth to a boil. Whisk in the flour mixture and simmer until thickened, 4 to 5 minutes. Remove from heat and stir in the mustard, salt, and parsley.
Meanwhile, using an ovenproof bowl or mug (about 4 inches in diameter), cut out 4 puff-pastry rounds to fit on top. Cut 3 slashes in the center of each round. Divide the filling among 4 ovenproof bowls and spoon some of the chicken-broth mixture over each. Top each bowl with a pastry round and bake until the pastry is puffed and golden brown, 25 to 30 minutes.