Adjust rack to middle position (top and middle positions for 8 servings) and preheat oven to 425 degrees. Bring a large pot of salted water to a boil. Wash and dry produce.

Trim and quarter zucchini lengthwise; slice crosswise into ¼-inch-thick quarter-moons.

Once water is boiling, add penne to pot. Cook, stirring occasionally, until al dente, 9-11 minutes. Reserve 1 cup pasta cooking water (2 cups for 8 servings), then drain. Keep empty pot handy for Step 4.

While pasta cooks, heat a drizzle of oil in a large pan over medium-high heat. Add beef*, zucchini, Italian Seasoning, garlic powder, a big pinch of salt, and pepper.

Cook, breaking up meat into pieces, until zucchini is tender and beef is cooked through, 5-7 minutes. (For 8 servings, work in batches, adding a drizzle of oil between batches.)

Remove pan from heat. TIP: If there’s excess grease in your pan, carefully pour it out.

Transfer beef mixture to pot used for pasta; add marinara, tomato paste, stock concentrates, and ½ cup reserved pasta cooking water (1 cup for 8 servings). Bring to a boil over medium-high heat; cook, stirring occasionally, until sauce is slightly thickened, 2-3 minutes. TIP: If sauce is too thick, stir in more reserved pasta cooking water a splash at a time.

Turn off heat; add drained penne to pot with sauce and stir to combine. Taste and season with salt and pepper if desired.

Transfer pasta to a lightly oiled 9-by-13-inch baking dish (use 2 lightly oiled 9-by-13-inch baking dishes for 8 servings). Carefully dollop ricotta all over pasta and top with mozzarella.

Bake, uncovered, on middle rack until cheese is melted, 10-12 minutes (for 8 servings, bake on top and middle racks, switching rack positions halfway through).

Increase oven temperature to broil; bake until cheese is browned, 2-3 minutes more. TIP: Watch carefully to avoid burning.

Let baked penne cool for 5 minutes before serving. Sprinkle with as many chili flakes as you like. Serve family style.