Step 1

Preheat oven to 400 degrees F (200 degrees C).

Step 2

Toss zucchinis and onions in a large bowl with olive oil until evenly coated. Spread vegetables into a large baking dish.

Step 3

Roast vegetables in the preheated oven until golden, about 30 minutes. Season with salt and pepper. Reduce oven heat to 350 degrees F (175 degrees C).

Step 4

Fill a large pot with lightly salted water and bring to a boil. Stir in penne and return to a boil. Cook pasta uncovered, stirring occasionally, until cooked through but still firm to the bite, about 11 minutes; drain.

Step 5

Lightly grease a 9x13-inch baking dish.

Step 6

Heat marinara sauce in a saucepan over medium heat until sauce is hot. Spread hot marinara sauce into the prepared baking dish, followed by penne pasta and zucchini mixture. Sprinkle Parmesan cheese atop vegetables.

Step 7

Bake in the preheated oven until sauce is bubbly, 35 to 40 minutes. Allow casserole to rest for 5 minutes before serving.