Preheat the oven to 350 F degrees.
Prepare the egg mixture: Crack the eggs in a large bowl. Pour in the milk and add in salt and pepper. Give it a whisk. Set aside.
To make the filling: Heat oil in a 12-inch nonstick skillet over medium heat. Add in the scallions and cook, stirring frequently until they are softened—2-3 minutes.
Stir in the garlic and cook for 30 seconds.
Add in the baby spinach and cook until wilted. Sprinkle it with the feta cheese.
Pour in the eggs. Using a spatula, gently stir and scrape the bottom of the skillet until large curds form but the eggs are still wet—about 30-45 seconds.
Smooth the top into an even layer and let it cook for 30 seconds.
Place it in the oven and bake until the center is no longer jiggly and edges are golden brown, about 8-9 minutes.
Meanwhile, make the topping: Mix tomatoes, onions, lemon juice, olive oil, parsley, salt and pepper in a bowl.
When the frittata is fully baked, carefully loosen the edges using a spatula and transfer it onto a cutting board. Let it cool for 5 minutes.
Garnish with the tomato mixture. Slice and serve.