Preheat oven to moderate.
Boil, steam or microwave pumpkin until just tender; boil, steam or microwave silverbeet until slightly wilted. Drain vegetables separately. Mash pumpkin in small bowl; reserve. Spread silverbeet over sheets of absorbent paper; cover with more absorbent paper, reserve.
Meanwhile, cook pasta in medium saucepan of boiling water about 5 minutes or until just tender; drain. Rinse under cold water; drain.
Heat oil in medium saucepan; cook leek, stirring, about 5 minutes or until softened. Add pumpkin and stock; cook, covered, stirring occasionally, about 5 minutes or until liquid is absorbed.
Meanwhile, combine silverbeet in medium bowl with ricotta and cream.
Spread pumpkin mixture over the bottom of 2-litre (8-cup) shallow baking dish; cover pumpkin mixture with half of the pasta. Spread pasta sauce over pasta; cover sauce with remaining pasta. Pour silverbeet mixture over pasta, gently spreading to completely cover pasta. Sprinkle pizza cheese all over surface of lasagne; place lasagne on oven tray. Bake, uncovered, in moderate oven about 30 minutes or until lasagne is heated through and cheese is browned lightly.
tip The lasagne can be assembled in advance and refrigerated overnight or frozen.