Start cooking 3-4 potatoes in water, depending on much mashed potato you want.
Heat the oil and cook the onion a few minutes, then add the ground beef, stirring to break up. Add the garlic. Cook until the beef is crumbly and browned (no pink left).
Stir in the ketchup and Worcestershire. Stir in the peas.
Combine the bouillon and water, then stir in the cornstarch. Pour this into the ground beef and simmer a few minutes. Turn the heat down and stir in the sour cream.
The potatoes should be nearly done, so drain and mash them (or put them through the ricer), then stir in butter and milk, salt and pepper to taste.
If your skillet can go under the broiler spread the potatoes on top of the meat and sprinkle the cheese on top, then broil a few minutes.
You can also spread the meat in a 9x9 baking dish, spread the potatoes on top, top with the cheese, and then bake or broil.
Notes
I'm not giving a recipe for the mashed potatoes because I never really measure. I do LOVE my potato ricer though and it's the only thing I use anymore to make mashed potatoes.