Stir pumpkin with ricotta, mascarpone, mozzarella, Parmesan, egg yolk, garlic and salt and pepper until smooth; set aside.
Sage Béchamel Sauce:
Melt butter in a saucepan set over medium heat. Add onions; cook for 5 to 7 minutes or until soft and lightly golden. Stir in sage, cook for 1 to 2 minutes or until fragrant.
Sprinkle in flour. Cook stirring, for 2 minutes or until smooth. Slowly whisk in milk until smooth. While stirring constantly, bring to a boil. Cook, stirring frequently, for 10 minutes or until sauce thickens slightly. Season with salt, pepper and nutmeg.
Cook pasta according to package directions and cool. Preheat oven to 375°F (190°C). Grease a 9x13-inch (3 L) baking dish. Pour half the Béchamel Sauce into the prepared pan. Divide filling evenly among cooked manicotti and arrange in pan. Pour remaining béchamel over top. Sprinkle evenly with Parmesan cheese.
Cover with foil and bake for 30 minutes. Remove foil. Uncover and continue to bake for 10 to 15 minute or until heated through.
Meanwhile, cook pancetta in small skillet set over medium heat for 6 to 8 minutes or until golden and crispy.
Sprinkle with crispy pancetta over top of pasta. Serve immediately.