Preheat your oven to 400F and set your rack to the center position.
Grease a pie dish with a pat of butter, taking care to get up the sides as well, and set aside.
In a frying pan, saute the slices of ham in a bit of olive oil over medium to medium-high heat just until the slices get some color. Remove from the pan and then cut the ham slices into bite-sized pieces and set them aside.
To the frying pan, add the sliced spinach and cook just until wilted and bright green. Add oil as necessary. Remove from the pan and set aside.
Now add the grape tomatoes, cut side down. Cook for about 30 seconds just to get some of the juice evaporated. Remove from the pan and set aside.
Whisk together the egg, milk, salt, and pepper. Do not over whisk, you just need the egg and milk combined. Over-whisking will result in a dense frittata.
Add the ham, spinach, tomato, and grated Pecorino Romano to the egg and just stir to combine.
Pour the egg mixture into the prepared pie dish and then carefully.
Bake for about 15-20 minutes or until edges are puffed and the center jiggles just slightly.
Remove promptly and set on a cooling rack. Slice and serve.