Boil the potatoes in salted water for 15 to 18 minutes until tender. Drain, mash with the 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. Bring just to a simmer and transfer the mixture to a buttered slow-cooker insert.
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 and the center reaches 165°F. Remove the lid and towel and rest for 10 minutes before serving.