Place the potatoes in a large soup pot or Dutch oven and fill with water just enough to cover. Bring to a boil and cook until tender, then drain. Add the yogurt, salt, and pepper and mash to your liking.
Heat the oil in a large non-stick skillet. Sauté the onions, garlic, and carrots over medium heat for 3-4 minutes.
Add in the mushrooms and fresh herbs. Cook stirring occasionally until mushrooms start to slightly brown.
Stir in the tomato paste and flour. Add the broth/stock little by little, stirring continuously until the sauce starts to form.
Taste and season with salt and pepper; allow the sauce to simmer for about 10 minutes, or until it fully thickens.
Meanwhile, preheat the oven to 350F.
Stir in the peas, and balsamic vinegar and transfer to a baking dish. Spread over the mashed potatoes and bake at 350F for 18-20 minutes or until bubbly and slightly brown on top.
Allow the shepherd's pie to cool for a bit before transferring to plates. Serve and enjoy!