Adjust rack to top position (middle and top positions for 4 servings) and preheat oven to 425 degrees. Wash and dry produce.

Dice potatoes into ½-inch pieces. Trim and thinly slice scallions, separating whites from greens; mince whites. Trim green beans if necessary.

Place potatoes in a large pot with enough salted water to cover by 2 inches. Bring to a boil and cook until tender, 15-20 minutes. Reserve ½ cup potato cooking liquid, then drain and return potatoes to pot.

While potatoes cook, in a large bowl, combine beef*, panko, scallion whites, half the garlic powder (you’ll use the rest later), and 1 TBSP water (2 TBSP for 4 servings); season generously with salt and pepper.

Form mixture into two 1-inch-tall loaves (four loaves for 4).

Place meatloaves on one side of a lightly oiled baking sheet. (TIP: Line sheet with foil first for easier cleanup.) Roast on top rack for 5 minutes.

Remove sheet from oven; carefully toss green beans on empty side of baking sheet with a drizzle of oil, salt, and pepper. Roast on top rack until meatloaves are cooked through and green beans are browned and tender, 12-15 minutes. (For 4 servings, leave meatloaves roasting; toss green beans on a second baking sheet and roast on middle rack.)

While everything roasts, trim and thinly slice mushrooms (skip if your mushrooms are pre-sliced!). Heat a drizzle of oil in a medium pan over medium-high heat. Add mushrooms; season with salt and pepper. Cook, stirring, until softened, 2-4 minutes. 

Stir in stock concentrate and ¼ cup water (⅓ cup water for 4 servings). Bring to a boil, then reduce to a low simmer. Cook until slightly thickened, 2-3 minutes. 

Turn off heat. Stir in half the sour cream (you’ll use the rest in the next step) and 1 TBSP butter (2 TBSP for 4) until melted and combined. Season with salt and pepper.

Heat pot with drained potatoes over low heat; add 1 TBSP butter (2 TBSP for 4 servings), remaining sour cream, and remaining garlic powder.

Mash until smooth and creamy, adding splashes of reserved potato cooking liquid as needed. Season with salt and pepper.

If necessary, reheat pan with sauce over low heat; stir until warmed through.

Divide meatloaves, mashed potatoes, and green beans between plates. Spoon sauce over meatloaves. Sprinkle with scallion greens and serve.