Brown the beef and onion for 10 minutes until the beef reaches 160°F, drain, and transfer to a 6-quart crockpot with the carrots and garlic.
Whisk the broth, tomato paste, Worcestershire, thyme, 1 teaspoon salt and pepper and pour it over the beef; cook on low for 5 hours.
Boil the potatoes until tender, drain, and mash them with the milk, butter and remaining salt.
Stir the frozen peas into the beef, spread the warm mashed potatoes over the filling, and cover.
Cook on high for 30 to 45 minutes until the peas are hot and the center reaches 165°F.