In a large saute pan on medium-high heat, melt 2 tablespoons butter and saute 1/2 of chopped onion, carrots and celery until softened. Add the meat and salt and cook on high, stirring occasionally until the moisture is evaporated and the meat is browning in fat, about 15 minutes.
When meat is browned, stir in Worcestershire and cook 1 minute. Stir in chicken stock and cornstarch-water mixture and simmer for additional minute to thicken.
Peel and halve potatoes. Place in pot and cover with cold water to 2 inches above potatoes. Add generous amount of salt. Bring to a boil and cook until tender, about 15-20 minutes. Drain potatoes and return to pot (or transfer to bowl). Mash the potatoes with 4 tablespoons butter, white pepper and a few tablespoons of milk until smooth. Add more milk if needed to make potatoes creamy.
In a 2-quart casserole dish, evenly distribute the cooked meat, top with peas and remaining chopped onion and dollop mashed potatoes on top. Dot top of potatoes with remaining 2 tablespoons butter.
Bake at 400 degrees Fahrenheit for 30 minutes, until warmed through and potatoes are golden on top, and pie is bubbling.