Grease an 11 by 7-inch glass baking dish or 16 oz round baking dish.
Add your potatoes to a large stockpot. Add cold water until water line reaches an inch above the potatoes. Add 1 teaspoon salt to the water. Turn the heat to high and cook until the water boils. Reduce the heat to medium-hight to maintain boil and cook until potatoes are fork tender, about 10 - 12 minutes. Drain.
Return potatoes back to warm pot and mash. Stir in the milk, butter and salt, don’t over mix. Taste and adjust seasoning. Set aside.
Place the oil in a 11- inch saute pan over medium heat, once the oil glistens, add the onion, carrots and celery and saute until they are fork tender, about 3 - 4 minutes. Add the garlic and stir to combine.
Add the meatless ground and the thyme, rosemary and sage and mix to coat. Sprinkle the plant-based meat with the flour and stir to coat. Add the dijon mustard, Worcestershire sauce, vegetable broth and water and stir to combine. Reduce the heat to low and let flavours blend and sauce slightly thickens, about 5 minutes.
Spread plant-based meat mixture evenly in the dish. Top with mashed potatoes and sprinkle with some fresh thyme. Place on a parchment lined half sheet pan on the middle rack. Bake for 25 minute or just until the potatoes begin to brown. Remove from oven and allow to cool for 5 minutes before serving.