In a skillet, heat oil & saute onions & garlic until translucent. Set aside to cool. In a large bowl, combine ground beef, salt, pepper, Worcestershire sauce, Parmesan, parsley, cooled onions & garlic, egg, breadcrumbs & milk. Combine well.
On 4 squares of waxed paper, shape into 4 patties with a 5-inch diameter. Shape each over an inverted custard cup; discard paper. Chill about an hour.
Mashed Potatoes
Peel & cook potatoes. In a large bowl, combine cooked potatoes, butter, seasonings, Parmesan & a splash of milk. Mash & add more milk gradually until potatoes are desired texture.
Cheese Sauce
In a saucepan, melt butter; whisk in flour, salt & pepper until smooth. Gradually whisk in milk. Bring to a boil; cook & stir while adding cheddar cheese. Cook, stirring constantly until thickened. Cook frozen peas.
Baking & Serving
Preheat oven to 375 F. Place inverted meat cups on a shallow baking dish. Bake for 20 minutes or until meat is cooked. Lift baked meat cups from custard cups & turn upright; fill with mashed potatoes. Place on serving plates, spoon cheese sauce over filled meat cups & top with green peas.