Heat the oven to 400°F and grease a 2 1/2-quart baking dish.
Heat the oil in a large skillet and cook the onion over medium heat until soft, about 6 minutes.
Stir in the tomato paste for 1 minute, then add the broth, Worcestershire and black pepper.
Fold in the pulled pork and frozen vegetables and simmer until the filling is hot and lightly thickened, 5 minutes.
Spread the pork mixture in the baking dish and spoon the warm mashed potatoes evenly over it.
Rake the potatoes with a fork, brush them with melted butter and bake until bubbling, 20 to 25 minutes.
Broil for 2 minutes if the ridges need more color, then rest the pie for 10 minutes.