Preheat oven to 375°F.
Bring large pot of salted water to boil. Boil pasta to al dente. Drain well.
In large pot, melt butter over medium heat. Whisk in flour. Reduce heat to medium-low and cook 1 minute, stirring once or twice to avoid burning. Gradually whisk in milk and raise heat to medium-high. Bring mixture to boil, whisking frequently until sauce has thickened, about 2-3 minutes. Remove from heat. Reserve 1/4 cup cheese. Add remaining cheese to sauce. Add mustard powder, cayenne, salt and pepper. Stir until cheese is fully melted. Stir in pasta and veggies.
Divide mac and cheese among six 8-ounce ramekins.
Unroll puff pastry and cut into 6 squares. Sprinkle remaining cheese over each ramekin top with puff pastry. Tuck sides of puff pastry down into ramekins. Brush tops of pastry with beaten egg.
Place ramekins on baking sheet and bake 35-45 minutes or until pastry is puffed and golden brown. Remove from oven and sprinkle with chopped parsley and salt. Serve immediately.