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1
Preheat oven to 375. Get out your spring-form pan, wherever it is, and lightly grease it. Put on a giant pot of water to boil for the pasta.
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2
Heat a tablespoon of olive oil in a large heavy skillet; raise heat to medium high and brown sausage well (without casing), mashing it to break it into very small pieces as it cooks, about 8-10 minutes.
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3
Place tomatoes, remaining half cup of olive oil, garlic in blender. Puree until just smooth. Once meat has browned properly, add puree to skillet. Add salt; bring sauce to a very lively simmer and cook, over medium high heat, for 5 minutes. Reduce heat and simmer until the sauce has reduced by about one-third (20 minutes). Add red pepper flakes, balsamic vinegar, and continue cooking for 5 more minutes.
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4
Meanwhile, mix together ricotta, half the Parmesan. Stir in the beaten egg. Thin the mixture with a bit of milk if it seems too thick for its own good. Season with freshly ground pepper (about 1/2 teaspoon).
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5
Cook the paccheri at a rolling boil until it is barely al dente. You don't want the large tubes to lose their shape and become too flattened. Rinse with cold water, then toss with a bit of olive oil and the remaining Parmesan cheese.
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6
Place the paccheri vertically in the prepared spring-form pan. Using a pastry bag or a small spoon, squirt about a tablespoon or so of the ricotta mixture into each noodle, pushing it down a bit to make room for the sauce. Pour the sauce over the cheese-filled noodles. Place the spring-form on a cookie sheet and bake for 25 minutes. Remove, cover the top of the casserole with slices of mozzarella, and continue cooking until the cheese has browned and the casserole has heated through, about ten more minutes. Serve in slices, like a pie.
