PREHEAT oven to 350°. Spray a 9-inch pie pan with cooking spray.
HEAT oil in a large skillet over medium-high heat. Add onions and cook, stirring occasionally, until onions are soft. Stir in spinach and continue cooking until excess moisture has evaporated.
COMBINE eggs, cheese, garlic powder, salt, pepper, and cayenne pepper in a large bowl. Add spinach mixture and stir to blend. Spread in prepared pie pan. Top with tomato slices, if using.
BAKE for 25 to 30 minutes or until eggs are set and knife inserted in the center comes out clean. Let cool for 10 minutes before serving. Cut into 6 slices.
