BBQ Beef & Mashed Potato Pie

Place the potatoes in a pot. Add cold water to cover; bring to a boil. Add 1 tsp salt; simmer until just tender, 15 to 18 minutes. Reserve 1/4 cup of the cooking liquid; drain the potatoes and return to the pot. Mash with 3 Tbsp oil, 1/4 tsp each salt and pepper and 2 Tbsp of the reserved cooking liquid (add more liquid if necessary). While the potatoes are cooking, heat the remaining Tbsp oil in a large skillet over medium heat. Add the onion and 1/4 tsp each salt and pepper and cook, covered, stirring occasionally, until tender, 8 to 10 minutes. Heat broiler. Add the beef to the onion and cook, breaking it up with a spoon, until no longer pink, 5 to 6 minutes. Spoon off and discard any grease. Add the tomatoes and cook, stirring occasionally, for 4 minutes. In a bowl, whisk together the ketchup, vinegar, mustard, molasses and Worcestershire; add to the beef and cook, stirring, for 1 minute. Transfer the beef mixture to a 2-qt broiler-proof baking dish and top with the mashed potatoes. Broil until beginning to brown, 2 to 4 minutes.