Adjust rack to top position and preheat oven to 450 degrees. Bring a medium pot of salted water to a boil. Wash and dry produce.

Halve, peel, and dice half the onion (whole onion for 4 servings). Finely dice tomatoes. Trim zucchini, then grate on the largest holes of a box grater.

Once water is boiling, add penne to pot. Cook, stirring occasionally, until almost al dente, 8 minutes (it’ll finish cooking in Step 6).

Drain penne. (Keep empty pot handy for Step 5.)

Meanwhile, heat a drizzle of olive oil in a large pan over medium-high heat. Add diced onion, tomatoes, and a pinch of salt. Cook, stirring occasionally, until onion is softened, 4-6 minutes.

Stir in marinara, zucchini, stock concentrates, tomato paste, ½ tsp Italian Seasoning, ¾ cup water, 1½ tsp sugar, 2 tsp olive oil, and ¾ tsp salt. (For 4 servings, use 1 tsp Italian Seasoning, 1½ cups water, 3 tsp sugar, 4 tsp olive oil, and 1½ tsp salt.) Bring to a simmer; cook, stirring, 3 minutes more. Turn off heat.

While sauce simmers, in a medium bowl, combine ricotta, ½ tsp Italian Seasoning (1 tsp for 4 servings), and a drizzle of olive oil. (Use the rest of the Italian Seasoning as you like.) Season with salt and pepper.

Add drained penne, sauce, and 1 TBSP butter (2 TBSP for 4 servings) to pot used for pasta. Season with salt and pepper; stir to thoroughly combine.

Spread out half the pasta mixture in an 8-by-8-inch baking dish. (For 4, use a 9-by-13-inch baking dish.) Using a tablespoon, dollop pasta mixture with herbed ricotta. Top with remaining pasta mixture, then sprinkle with mozzarella.

Cover baking dish with foil. (TIP: Coat inside of foil with nonstick spray first to prevent sticking.) Bake pasta on top rack for 10 minutes, then remove from oven; discard foil.

Heat broiler to high. Broil until cheese is lightly browned, 4-5 minutes. TIP: Watch carefully to avoid burning.

Let cool slightly; top with chili flakes if desired. Divide between plates and serve.