Cook lasagne according to packet directions. Drain, pat dry and keep moist in a damp towel OR spray each sheet with cooking spray OR lightly apply some oil.
In a separate large frypan over medium heat, combine the onions with the butter and saute for 5 minutes.
Prepare the instant mashed potatoes according to packet directions, but omit the milk. Stir in the cream cheese until well blended.
Place 3 sheets lasagne in the bottom of a lightly greased 20x30cm baking dish. Spread 1/2 the potato mixture over the pasta in the dish. Top this with 3 more sheets, followed by the other 1/2 of the potato mixture. Finish by topping with the remaining 3 sheets then top those with sauteed onions.
Bake at 180 degrees C for 20 minutes, or until bubbly. Allow to cool for 5 minutes before cutting.