Preheat your oven to 375°F (190°C). While that's happening, bring a large pot of salted water to a boil for your pasta.
Cook the pasta for TWO MINUTES LESS than the package directions state. You want it very al dente (almost crunchy) since it will continue cooking in the oven. Drain and toss with 1 tablespoon of olive oil to prevent sticking. Set aside.
Brown the sausage in a large skillet over medium-high heat, breaking it up with a wooden spoon as it cooks. This should take about 5-7 minutes. Don't rush this step—those caramelized bits are pure flavor.
Add the onion and bell pepper to the sausage and cook for another 3-4 minutes until they start to soften.
Toss in the mushrooms (if using) and cook for 2-3 minutes until they release some moisture and begin to brown.
Add the garlic, Italian seasoning, and red pepper flakes, and cook for another 30 seconds until fragrant. Your kitchen should smell amazing right about now.
Pour in the marinara sauce and chicken broth, stirring to combine everything. Let it simmer for about 5 minutes to bring the flavors together.
Remove from heat and stir in the cooked pasta, 1 cup of mozzarella, ¼ cup of Parmesan, and the fresh basil. Season with salt and pepper to taste.
Transfer everything to your baking dish and spread it out evenly. Top with the remaining cup of mozzarella and ¼ cup of Parmesan.
Bake uncovered for 20-25 minutes until the cheese is bubbly and starting to brown in spots. If you want an extra golden top, broil for the last 2-3 minutes (but watch it like a hawk—the difference between "beautifully browned" and "charcoal" is about 37 seconds).
Let it rest for 5 minutes before serving. This cooling time isn't just to prevent mouth burns—it also allows the sauce to thicken and the flavors to settle.
Garnish with additional fresh basil and serve hot, preferably with garlic bread for sauce-mopping purposes.