Boil the potatoes in salted water for 15 to 18 minutes until tender. Drain, reserve 1 teaspoon of the butter for the slow-cooker insert and mash with the remaining butter, warm milk, 1/2 teaspoon salt and 1/4 teaspoon pepper, and keep warm.
Heat the oil in a skillet. Cook the onion and carrots for 7 minutes, then stir in the tomato paste for 1 minute.
Add the gravy, cooked pot roast, peas, remaining salt and pepper. Reheat the filling in the skillet until it reaches 165°F throughout, then transfer it to a slow-cooker insert greased with the reserved teaspoon of butter.
Spread the warm mashed potatoes evenly over the beef. Cover with a clean kitchen towel under the lid to catch condensation.
Cook on high for 60 to 75 minutes, until the filling bubbles at the edge. Remove the lid and towel and rest for 10 minutes before serving.
