Preheat your oven to 375 degrees F. Crack all 10 eggs into a mixing bowl, add the milk, 1/4 teaspoon of the salt, and the black pepper. Whisk until the yolks and whites are fully combined and slightly foamy on top, about 45 seconds of good whisking. Set aside.

Heat the olive oil in a 10-inch oven-safe skillet over medium heat. Add the diced onion and bell pepper with the remaining 1/4 teaspoon salt. Cook for 4 to 5 minutes, stirring occasionally, until the onion is soft and translucent and the pepper has softened.

Add the minced garlic and dried oregano and cook for 30 seconds until fragrant. Add the spinach and stir it into the vegetables. It will wilt down in about 60 seconds. Spread the vegetable mixture evenly across the bottom of the skillet.

Run a silicone spatula around the edges of the skillet to make sure nothing is sticking. Scatter the crumbled feta evenly over the vegetables. Pour the whisked egg mixture directly over the top. Tilt the pan gently so the eggs settle into the vegetables evenly. Let the frittata cook on the stovetop undisturbed for 2 to 3 minutes until the outer edge begins to look set and opaque while the center is still jiggly.

Transfer the skillet carefully to the preheated oven. Bake for 12 to 15 minutes. The frittata is done when the center no longer jiggles when you nudge the pan and the top looks matte rather than wet and shiny. Do not overbake or the eggs will be rubbery. A slightly domed top is a good sign.

Remove from the oven with a dry oven mitt and let the frittata rest in the pan for 3 minutes before slicing. Run the spatula around the edge once more, then slice into wedges directly in the skillet. Garnish with fresh parsley if you like and serve straight from the pan.