To save time, and ensure that the potatoes are still warm when ready to use, boil the potatoes while you prepare the filling in step 2. Place the potatoes in a large saucepan or stockpot, and add enough water to cover, so that the water line is about an inch above the potatoes. Bring to a boil over high heat, then reduce heat to medium-high and boil until soft, about 10–15 minutes, depending on the size of your potatoes. Drain the potatoes and set aside.

Heat the olive oil in a large skillet over medium heat. Add the onions, carrots, celery, and mushrooms and cook, stirring occasionally, until the vegetables begin to soften, about 5 minutes. Add the garlic and cook, stirring, for 1 minute. Add the red wine and vinegar to the pan. With a silicone spatula or wooden spoon, stir and scrape up any vegetable bits sticking to the pan. Cook until most of the liquid has evaporated, about 4 minutes. Add the tomato paste, broth, beans, herbs, salt, pepper, and bay leaf and let the mixture come to a boil. (You may need to increase the heat to medium-high to help it along.) Once it starts to bubble, reduce the heat to medium-low and simmer, stirring occasionally, until the liquid has reduced and thickened into a sauce, about 20 minutes. Remove the skillet from heat. Remove and discard the bay leaf, and then stir in the frozen vegetables. If your skillet is not oven-safe, transfer the filling to a greased 2.5–3-quart baking dish.

Preheat oven to 350°F (177°C).

Place the warm potatoes in a large mixing bowl. Add the butter, sour cream, and salt. With a handheld potato masher or electric mixer, mash or beat until combined and creamy. Fold in the shredded cheese. Spread the mashed potato topping evenly over the filling.

Bake for 30 minutes; then, keeping the shepherd’s pie in the oven, turn on the broiler setting and broil on high for 1 to 3 minutes, until golden spots appear on the topping.

Remove from the oven and allow to cool for 5 minutes before serving. Sprinkle with fresh parsley, if desired, and serve warm.

Leftovers keep well in the refrigerator in an airtight container for up to 5 days. Reheat as desired. You can also freeze this for up to 3 months. Thaw in the refrigerator and reheat as desired.