Adjust rack to middle position and preheat oven to 425 degrees. Wash and dry produce.

Halve, peel, and dice onion into ½-inch pieces. Trim and halve zucchini lengthwise. Lay flat and slice into ¼-inch-thick planks; reserve 6-8 of the widest zucchini planks (you’ll use them to top the “lasagna” before baking). In a shallow dish, toss remaining zucchini planks with half the flour until coated.

Heat a drizzle of oil in a large, preferably nonstick, pan over medium-high heat. Add beef*, onion, garlic powder, Italian Seasoning, salt, and pepper. Cook, breaking up meat into pieces, until cooked through, 4-6 minutes. Sprinkle with remaining flour and cook, stirring, until evenly combined, 30-60 seconds.

Add crushed tomatoes and stock concentrates; cook, stirring constantly, until mixture has thickened, 1-2 minutes more.

Meanwhile, in a medium bowl, combine ricotta, half the Parmesan, one-quarter of the Italian cheese blend, salt, and pepper.

Arrange half the coated zucchini planks in a single layer on the bottom of an 8-by-8-inch baking dish (for 8 servings, use a 9-by-13-inch baking dish or two 8-by-8-inch baking dishes).

Spread half the cheese mixture in an even layer over zucchini, then top with one-third of the remaining Italian cheese blend and half the ragù. Repeat one more time with remaining coated zucchini planks, remaining cheese mixture, another one-third of the remaining Italian cheese blend, and remaining ragù.

Top “lasagna” with remaining Italian cheese blend, reserved zucchini planks, remaining Parmesan, and a drizzle of olive oil.

Bake “lasagna” on middle rack until zucchini is tender and cheese is browned, 35-40 minutes.

Let cool for at least 5 minutes.

Cut “lasagna” into four pieces (eight pieces for 8 servings); divide between plates and serve.