Preheat the oven to 350°F and grease a glass baking dish.
Use a pat of the butter in the bottom of a skillet to brown the chopped onion, garlic, and mushrooms. Add the ground meat and cook until lightly browned, then add the stock, thyme, and chopped tomatoes and cook until the tomatoes are soft. Remove any fat. Season to taste with salt, pepper, and a little Worcestershire sauce. Remove the meat mixture to the glass baking dish. Mix the flour with a little water to make a paste to add to the cooked meat liquid to thicken. Whisk well and add to casserole dish, blending well.
Mix salt, egg yolk, and remaining butter into the mashed sweet potatoes. Spoon into a pastry bag with a star point. Pipe the sweet potato mixture over the casserole.
Bake for 25 to 30 minutes until lightly browned.