Heat the butter and oil in a 10-inch skillet over medium heat until hot. Add
the tortilla strips and onion; cook, stirring occasionally, until the tortillas
Mix the remaining ingredients except the cheese and pour into the skillet. As
the mixture begins to set at the bottom and sides, gently lift the cooked portion
so that the uncooked eggs can run under the cooked portion. Avoid constant stirring.
Turn the egg mixture, cook until the eggs are all cooked through but still moist,
3 to 5 minutes.