Preheat oven to 400°.
Cook the potato in boiling water 5 minutes or until tender; drain.
Combine egg whites and eggs in a large bowl, stirring with a whisk. Stir in potato, Canadian bacon, salt, pepper, and spinach.
Coat a 9-inch cast-iron skillet with cooking spray; place over medium-high heat. Add onion; sauté 4 minutes or until tender. Add potato mixture to pan; cook over medium heat 5 minutes or until almost set. Sprinkle with cheese. Bake at 400° for 6 minutes or until set. Let stand 3 minutes; cut into 4 wedges.