3 zucchinis, sliced
2 onions, sliced
3 tablespoons olive oil
salt and ground black pepper to taste
1 (16 ounce) package penne pasta
1 (28 ounce) jar marinara sauce
1/2 cup grated Parmesan cheese
Preheat oven to 400 degrees F
Toss zucchinis and onions in a large bowl with olive oil until evenly coated. Spread vegetables into a large baking dish.
Roast vegetables in the preheated oven until golden, about 30 minutes. Season with salt and pepper.
Reduce oven heat to 350 degrees F
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.
Lightly grease a 9×13-inch baking dish.
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.
Bake in the preheated oven until sauce is bubbly, 35 to 40 minutes. Allow casserole to rest for 5 minutes before serving