In small bowl, beat together eggs, milk and onions until blended. In 7- to 8-inch omelet pan or skillet over medium heat, heat butter until just hot enough to sizzle a drop of water. Pour in egg mixture.
As egg mixture begins to set, gently draw an inverted pancake turner completely across bottom and sides of pan, forming large soft curds. Continue until eggs are thickened and no visible liquid egg remains. Do not stir constantly. Spoon eggs down center of tortilla. Top with taco sauce and cheese. Fold or roll.