Sauté: Preheat oven to 350°F (180°C). Heat oil in a skillet over medium heat. Sauté onion, garlic, thyme, rosemary, and half of the salt/pepper for 5–7 minutes until fragrant.
Brown & Simmer: Crumble the ground beef into the skillet and cook until browned. Sprinkle with flour and stir. Add the ketchup, tomato juice, Worcestershire sauce, and mustard. Simmer for 5 minutes until the sauce thickens slightly.
Mix: Fold in the frozen mixed vegetables and transfer the entire mixture into a 9-inch baking dish.
Whip the Topping: In a separate bowl, whip the warm mashed potatoes with the warm milk, butter, minced garlic, and the remaining salt and pepper until fluffy.
Layer: Spread the potato mixture evenly over the meat, using a fork to create little "peaks" (these will get extra crispy).
Bake: Bake for 35 minutes or until the potato topping is golden and the filling is bubbling.