Heat the butter and oil in a large skillet over medium heat and saute the onions, carrots, celery and garlic until tender, 7 to 10 minutes. Season with salt and pepper.
When the vegetables start to brown, mix in the tomato paste. Add the beef and cook until no longer pink, about 10 minutes.
Add the Worcestershire and stock, season and simmer 10 minutes. Mix in the peas.
Spread in an oven-proof baking dish and cover with the potatoes, then rake with a fork to make peaks that brown nicely.