Bring a large pot of liberally salted water to a boil over high heat.
Cook the rigatoni pasta for one minute less than the package directs.
Drain the pasta and set aside (add a drizzle of olive oil, if desired, to prevent sticking).
Heat the olive oil in a large pot over medium-high until shimmering.
Break up the sausage and cook (stirring) until no longer pink (approx. 5 minutes).
Add the fennel, eggplant, salt, garlic powder, and crushed red pepper, and continue to cook until the vegetables are tender (approx. 5 to 10 minutes).
Pour in the tomato sauce and toss to combine.
Preheat the oven to 350 degrees F.
Stir in the cooked pasta and transfer about half the mixture to an oven-safe baking dish.
Top with dollops of mascarpone.
Layer on the remaining rigatoni mixture, and sprinkle on the mozzarella.
Bake until the cheese is melted and the sausage rigatoni is warmed through to the center (approx. 10 to 25 minutes).