Preheat the oven to 450 degrees F.: The oven should feel like a hot blast when you open the door, delivering dry heat that encourages browning and caramelization. Preheating to a high temperature enables the vegetables to sear at the edges quickly, locking in flavor. You may notice the air in the oven shimmer slightly when it reaches temperature. A common mistake is not waiting for the oven to fully preheat, which can result in steamier, less caramelized vegetables; be patient and allow the oven to reach the full temperature before roasting.
Spray a large baking sheet with oil, toss the peppers, zucchini, fennel, asparagus, red onion and garlic with balsamic, olive oil, 1/2 teaspoon salt, black pepper, and dried herbs. Roast until tender on the lower rack of the oven, about 15 to 20 minutes stirring halfway, until slightly browned tender.: When the vegetables hit the hot sheet they should sizzle softly, and after about 10 minutes you will hear gentle pops or feel faint steam as moisture escapes. The edges should develop golden to mahogany patches, signaling caramelization. The aroma will shift from raw vegetal to warm and slightly sweet, with roasted garlic notes rounding everything out. Stirring halfway encourages even color, so use a spatula to turn pieces to expose new surfaces to heat. Avoid overcrowding the pan, which can trap steam and prevent browning; if veggies are crowded, they will steam rather than roast, yielding limp results.
Meanwhile, bring a large pot of salted water to a boil and cook the pasta according to package directions for al dente, (undercooking slightly as it will continue cooking in the oven), drain.: The pot should sing as it boils and the water should have rolling movement, not a sleepy simmer. Add salt until the water tastes like the sea, which seasons the penne internally. Cooking to just under al dente gives the pasta a slight resistance so it will finish cooking in the oven without going mushy. After draining, the pasta should feel firm but yielding, with a toothsome center. A typical misstep is overcooking the pasta, resulting in a softer texture after baking; check a piece a minute or two before the shortest time on the package.
In a medium bowl, combine the ricotta, parsley, 6 tablespoons of the Pecorino Romano cheese and half of the mozzarella cheese.: As you mix, the ricotta should feel cool and slightly grainy, contrasted by the saltier Pecorino Romano that adds tangy granularity. The chopped parsley brings flecks of green and fresh herbal brightness, cutting through the dairy richness. Mixing creates a creamy, cohesive layer that will sit between the pasta and vegetables and prevent the casserole from becoming dry. A common pitfall is under seasoning this layer; taste and adjust with a pinch of salt if needed before assembling.
Spray a deep 13 x 9-inch baking dish with olive oil spray.: This creates a nonstick surface and encourages the bottom edges to crisp slightly during baking. The dish should be deep enough to hold layers without spilling when you transfer to the oven. A warm dish straight from the fridge can cause uneven heating, so use a room temperature dish for predictable baking. Skipping the spray increases the chance of the casserole sticking, making serving messy and reducing the nice browned edges.
Put half of the cooked roasted vegetables into the prepared baking dish, then half of the pasta and top with 1 cup of the sauce. Spoon the ricotta mixture on top of the sauce in an even layer. Cover with the remaining pasta, then vegetables, then the remaining sauce. Finish with the remaining mozzarella and 2 tbsp Pecorino Romano cheese.: You will feel a pleasing weight as you layer, and the colors will alternate between reddish sauce and the warm tones of roasted vegetables. Spoon the ricotta gently so it spreads into an even cushion without clumping, creating contrast between cool creamy pockets and bubbling sauce. The layered structure traps steam and melds flavors while the top cheeses brown and crisp. Avoid piling ingredients haphazardly, which can cause uneven baking; take a moment to press layers lightly for an even surface so the casserole heats consistently.
Cover with foil and bake in the center of the oven for 25 minutes, or until the mozzarella is melted.: As it bakes, you will hear subtle bubbling from the sauce and smell a vivid mix of tomato, roasted garlic, and melted mozzarella . The foil traps moisture, ensuring the interior becomes hot and cohesive, while the final few minutes after removing foil allow the top to brown. A golden, bubbling top and gentle jiggle in the center are the visual cues that it is done. Removing foil too early can dry the surface, while leaving it on too long can prevent the cheese from browning; time it carefully and check after 20 to 30 minutes.