Place pumpkin and ¾ cup water in large skillet over medium heat. Cover and steam until tender, 20 minutes. Place steamed pumpkin in medium bowl and mash until smooth.
Heat 1?½ teaspoons olive oil in a small skillet, and sauté garlic until golden. Transfer garlic to mortar and pestle, and crush into a paste. Stir cheeses, chopped sage, salt, pepper and garlic paste into mashed pumpkin. Set aside.
Bring a large pot of water to a boil. Cook lasagna sheets until tender, about 2 minutes. Transfer to a plate and drizzle with olive oil to prevent pasta from sticking together. Reserve ¼ cup pasta water.
Liberally brush a medium baking dish with oil. Place a lasagna sheet on a clean work surface. Add four tablespoons pumpkin mixture to center of lasagna, so that it can be folded over to form a cannelloni tube. Transfer to prepared baking dish. Repeat with remaining lasagna sheets, laying them side by side in the pan.
Pour reserved pasta water over lasagna and cover dish tightly with foil. Bake in preheated oven until heated through and pasta is tender, 20 to 25 minutes.
Cook butter and sage leaves in small skillet over medium-high heat until golden-brown. Drizzle over cannelloni and serve immediately.