Bring a large pot of water to a boil. In a large saute pan, heat enough olive oil to coat the bottom of the pan. Remove the casing from the 2 sausage links and break into bite-size pieces (sausage can be sweet or hot, whichever you prefer.) Drop into the pan and begin browning over medium to medium high heat. While sausage is browning, dice the eggplant into similar size pieces as the sausage (I do about a 3/4 dice). When sausage is about half-way cooked, add eggplant, mushrooms, and garlic, and stir through. You want to have a similar amount of eggplant and sausage, so if your eggplant is a little too big, you can dice less and save the rest for another dish. Or if you really freaking love eggplant, put it all in there.
Add the dried rigatoni to the boiling water and cook 3 minutes short of package directions for al dente preparation. While pasta is cooking, saute the sausage and vegetables over medium to medium-high heat for about 10 minutes, until sausage is thoroughly cooked, testing vegetable doneness as you go. The mushrooms should be cooked through but still retain some bite, and the eggplant should be soft, but not mushy. When all is cooked through, add the jar of sauce to the mixture, stir through, and remove from heat.
At this point, put the ricotta into a large bowl and add 1/4 C milk, garlic powder, oregano, salt and pepper, and crushed red pepper (if desired.) Stir through vigorously until milk and cheese are combined and the cheese has a creamy, loose consistency similar to that of yogurt or well-prepared grits. If the cheese still feels a bit thick, add another 1/4 C milk and repeat.
After the pasta is cooked, drain well and dump noodles into a 13 x 9 Pyrex baking dish, lubed well with olive oil. Using a rubber spatula or plastic serving spoon, cover the pasta with the ricotta mixture and spread to the edges. Don't worry about having a consistent layer across, but try not to leave any large gaps that have no cheese. Sprinkle about 2 oz of the mozzarella and all of the parmigiana cheese over the ricotta. Then, add the contents of the saute pan : the sauce, sausage, mushrooms, garlic, and eggplant and spread this across the surface of the noodles and cheese, again, as evenly as possible. Top with the remaining 6 ounces of mozzarella and cover with aluminum foil. Bake for 20 minutes at 450 F until the mozzarella on top is fully melted and the sauce is bubbly. Let stand 10-15 minutes before serving.