Heat oil in a pan over medium heat. Add onions, garlic, and beef. Cook and stir until beef is browned and crumbly, 5 to 7 minutes. Add stock, peas, carrots, tomato paste, parsley, salt, and pepper. Reduce heat and cover. Simmer until carrots are tender, about 20 minutes.
While beef mixture is cooking, place potatoes into a large pot and cover with salted water. Bring to a boil. Reduce heat to medium-low and simmer until tender, about 20 minutes. Drain. Add Cheddar cheese and butter to the pot and mash together with the potatoes.
Preheat the oven to 400 degrees F (200 degrees C).
Spoon beef mixture into a baking dish. Top with potato-cheese mash.
Bake in the preheated oven until golden brown on top, about 30 minutes.