Preheat oven to 350 degrees Fahrenheit. Spray an 8 x 12 or equivalent round baking dish with nonstick cooking spray.
Slice and peel potatoes, and chop into large pieces. Place in a medium pot of salted water and boil until tender. Drain completely, then return to pot. Add milk, sour cream, garlic salt, and tender. Mash until very smooth. You can even use a hand mixer to make the potatoes extra creamy. Set aside.
In a large pot, heat olive oil. Add onions and mushrooms and cook until tender. Add flour and mix to coat mushrooms and onions.
Pour red wine and broth into the pot and bring to a simmer. Add mixed vegetables, thyme, sage, garlic powder, pepper, and salt. Bring to a simmer for at least 10 minutes, stirring frequently, and allowing the mixture to thicken.
Pour mushroom mixture into the prepared dish. Carefully scoop mashed potatoes on top of the mixture. I used an ice cream scoop for consistency but you can use a regular spoon.
Bake for 25-30 minutes until the potatoes begin to brown and mixture is bubbling.
Serve warm. Enjoy!
Recipe by For the Love of Gourmet at https://fortheloveofgourmet.com/vegetarian-shepherds-pie/