Preheat oven to 375°F. Lightly oil or butter a 1 1/2-quart baking dish. In a large saucepan, boil potatoes and 2 cloves garlic in salted water until tender, about 15 minutes. Drain potatoes and garlic, return to saucepan and mash with a potato masher. Stir in Silk, butter or olive oil, salt and pepper. Taste and adjust seasonings.
While potatoes are cooking, place a large skillet over medium heat. Add oil to hot pan and brown the ground beef, about 3 minutes. Add onion, carrots, celery and sauté until onions begin to soften, about 5 minutes. Add mushrooms, garlic, thyme, bay leaf and tomato paste, stirring until liquid reduces, about 5-7 minutes. Sprinkle flour over mixture and stir for 1 minute. Slowly add Silk, stirring continuously until completely combined. Add beef stock and Worcestershire, simmering for 3-4 minutes. Remove bay leaf and season with salt and pepper.
Spoon beef mixture into prepared baking dish and top with mashed potatoes, leaving a half-inch border around the edges. Bake for 20-30 minutes, or until the potatoes are brown and juices bubble around the edges. Allow to cool 10 minutes before serving.