Separate egg whites from yolks. Reserve the yolks for later use. Pour the egg whites into a bowl and beat until frothy. Add water to make the mixture runny. Add bacon, cheese and onion and mix well. Add salt and pepper to taste. Bend each asparagus stalk until it breaks and discard the tough stalk end. Cut the other ends into 1-inch pieces. Blanche asparagus pieces in a pot of boiling water for just a couple of minutes so they are still crisp/tender. Drain asparagus and fill the pot with cold water to stop the cooking process. Heat a 9-inch skillet over medium heat. When skillet is nice and warm, spray with non-stick spray and add 1/8 to 1/6 of the omelet mixture, depending on how many omelets you plan to make. Add 4 to 5 pieces of the asparagus into the omelet mixture in the pan. Cover the pan with a lid and cook for about 3-5 minutes depending on the heat of the stove. Lift lid to check the omelet; it should be nice and puffy and starting to firm up but still a little moist on top. Fold omelet in half with a spatula and continue to cook, flipping occasionally, until the entire omelet has firmed up. Repeat for each omelet.