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. Cut broccoli into bite-size pieces 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.) Toss broccoli on empty side with a drizzle of oil; season generously with salt and pepper.

Roast on top rack until meatloaves are cooked through and broccoli is browned and tender, 15-20 minutes. (For 4 servings, use 2 baking sheets; roast meatloaves on middle rack and broccoli on top rack.) TIP: If broccoli finishes first, remove from sheet and continue roasting meatloaves.

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 broccoli between plates. Spoon sauce over meatloaves. Sprinkle with scallion greens and serve.