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

Trim, peel, and cut carrots on a diagonal into ½-inch-thick pieces. Trim and halve zucchini lengthwise; slice crosswise into 1-inch-thick half-moons.

In a medium bowl (use a large bowl for 4 servings), gently combine beef*, panko, garlic powder, salt (we used ¾ tsp; 1¼ tsp for 4), and pepper.

While everything roasts, halve, peel, and thinly slice onion. Pick parsley leaves from stems; roughly chop leaves.

Heat a drizzle of oil in a large, preferably nonstick, pan over medium-high heat. Add zucchini; cook, stirring, until softened and lightly browned, 5-7 minutes (7-10 minutes for 4 servings). Turn off heat; transfer to a plate. Wipe out pan.

Add a large drizzle of oil to same pan over medium-high heat. Add onion; cook, stirring occasionally, until browned and softened, 6-9 minutes. TIP: Lower heat and add a splash of water if onion begins to brown too quickly.

Add cream cheese, stock concentrate, ½ cup water, and ¾ tsp sugar (1½ tsp for 4 servings) to pan with onion; cook until thickened, 1-3 minutes more.

Turn off heat; stir in 1 TBSP butter (2 TBSP for 4) and a pinch of chopped parsley. Taste and season with salt and pepper.

Carefully toss zucchini and carrots together with half the sumac (all for 4 servings).

Divide meatloaves and carrots and zucchini between plates. Spoon caramelized onion sauce over meatloaves; garnish with remaining chopped parsley. Serve.