1. Preheat oven to 350°F. Coat 10-inch springform pan with cooking spray. Bring large pot of salted water to a boil with fresh sage sprigs; use this boiling water to separately blanch mushrooms (3 minutes), then leeks (2 minutes), and then broccoli (1 minute). Immediately transfer each vegetable to colander with slotted spoon, rinse under cold water, drain well, and transfer to 1 large bowl.
2. Cook rigatoni in boiling water 3 minutes less than cooking time recommended on package, then drain, rinse under cold water, and transfer to separate large bowl.
3. Melt butter in large saucepan over medium heat. Add flour, and cook 3 to 5 minutes, or until beginning to turn light brown, stirring constantly. Add butternut squash soup, and cook 5 minutes, or until thickened, stirring constantly. Stir in 1 cup grated cheese, chopped sage, and 1 tsp. truffle oil (if using); season with salt and pepper, if desired. Stir soup mixture into pasta, add blanched vegetables, and stir to combine.
4. Spread one-third of pasta mixture in prepared springform pan; dot with 1/3 cup ricotta cheese. Repeat twice with remaining pasta mixture and ricotta, and then sprinkle with remaining 1 cup grated cheese. (If making ahead, cover springform pan tightly with foil, and refrigerate up to 24 hours. Uncover before baking.)
5. Place springform pan on large baking sheet, and bake 30 minutes, or until top is browned and cheese is melted. Let stand 5 minutes before unmolding and serving.