While oven is heating, heat a 10.5” to 12” (26 cm to 30 cm) oven-safe skillet over medium-high heat. (Note: skillet size is for 4 servings. If customizing, use a larger or smaller skillet, or you can transfer the cooked filling to a muffin tin and top it with the egg mixture before baking.)
To heated skillet add cooking oil and then ham, spinach, and green onions until spinach is wilted, 5 to 7 minutes.
Pour egg mixture over vegetables and ham, stirring to combine.
Transfer skillet to oven and bake for 15 to 17 minutes. (Frittata is done when eggs are set and a knife inserted in the center comes out clean.)
Slice frittata and serve with tomatoes and extra green onions on top. Enjoy!