Preheat oven to 350° F.
Heat oil in 10-inch skillet with a heat-resistant or removable handle. Add onion, sauté until onion is tender and golden-brown — will take about 5 minutes.
In a large bowl, combine remaining ingredients with whisk, beating until well-blended. Turn into skillet with onion.
Cook over low heat–lifting gently from bottom with a spatula as the eggs set — about 3 minutes.
Bake, uncovered, for 10 minutes or until top is set. With spatula, loosen from bottom and around edge, and slide onto a serving platter.
Cut into wedges.