Combine first 6 ingredients in a large bowl; stir with a whisk. Stir in potatoes and spinach.
Melt butter in a large ovenproof skillet coated with cooking spray over medium heat. Add egg mixture to pan, spreading evenly. Cook over medium-low heat 5 minutes or until center is almost set; sprinkle with cheese.
Broil 7 minutes or until top is golden brown. Cut into wedges, and top with marinara sauce.