In an extra large skillet set over medium heat, add the oil. Brown meat. When meat is almost browned, add the carrot and celery, saute until soft, about 5 minutes. Add the onion and continue cooking for 2 minutes. Add the garlic and cook for 1 minute.
Sprinkle in the flour and toast for 30 seconds, then mix in the tomato paste and cook for 1 minute. Gradually pour in the beef stock, whisking to create a smooth sauce.
Add the Worcestershire sauce, vinegar, garlic powder, salt, pepper, cumin, thyme, and bay leaf. Bring to a low simmer and cook for 15 minutes, stirring occasionally.
Add peas and stir. Cook for an additional 3 minutes, then remove from the heat.
Meanwhile, in a large stockpot filled with water, boil the potatoes until fork-tender, about 20 minutes.
Drain potatoes and return the pot to the burner. Let the heat evaporate any excess moisture. This should take a minute or so. Add the cream, butter, salt, pepper, and cheddar, if desired. Using a potato masher, mash potatoes into a smooth, creamy consistency. They should be on the thicker side.
Preheat oven to 375°F.
Pour the filling into a 9x13-inch baking dish or keep in skillet if it's deep enough to hold the potato layer. Using a spatula (or an offset spatula), carefully spread the potatoes over top. You will want to add "dollops" of potatoes in spots so that the weight of the potatoes doesn't drag filling into them.
Use a fork to drag swirls or marks into the layer of potatoes. Drizzle with melted butter.
Place in the oven and bake for 40 minutes. If the top is not as gold as desired, turn the broiler on low and broil for 5 minutes or until golden brown.
Remove from the oven and allow to rest for 20 minutes before serving.