Preheat oven to 425°F (220°C).
In a large bowl, combine eggs, half and half, cheese, Italian herbs, green onions, salt, and pepper. Set aside.
In a 10-12-inch skillet or cast iron skillet, over medium-high heat, add a little oil and sauté veggies: first onion until softened, and then add mushrooms. Add garlic and saute for 30 seconds. Stir in spinach and cook until it's wilted and reduced in size. Reduce the heat to medium-low.
Give the beaten eggs mixture a good stir and pour over the vegetables, covering them evenly. You can gently use a spatula to evenly spread out the eggs and cheese. Top with tomatoes tucking them in halfway through. Cook for a couple of minutes without stirring so it can set a little.
Bake for 15-20 minutes in preheated oven or until egg has completely set all around, veggies are cooked, and edges are golden brown. You can turn the broiler setting for the last 5 minutes for the top to brown better.
Take out of the oven and let it sit for a 2-3 minutes (or longer) before slicing. The frittata should set and look like quiche. The longer it sits and cools the easier you can cut it.
Top with fresh parsley and enjoy!