Heat the oven to 400°F (200°C), and prepare a well-seasoned or nonstick 10-inch ovenproof skillet.

Whisk the eggs, milk, kosher salt, black pepper, and optional oregano together just until blended. Avoid overbeating the eggs.

Warm 1 tablespoon of olive oil in the skillet over medium heat. Add the diced onion and cook for 3–5 minutes, until softened.

Add the minced garlic and cook for about 30 seconds, stirring constantly so it becomes fragrant without browning.

Add the chopped spinach in batches and cook for 1–2 minutes, until just wilted. If the spinach releases substantial liquid, continue cooking briefly until the vegetables are fairly dry.

Spread the onion, garlic, and spinach evenly across the skillet. Add the remaining tablespoon of olive oil if the pan looks dry, then pour in the egg mixture.

Cook the frittata on the stovetop for 2–3 minutes without stirring, until the edges begin to set.

Sprinkle the crumbled feta evenly over the surface. Transfer the ovenproof skillet to the preheated oven.

Bake for 10–15 minutes, until the center is just set and the top is lightly puffed. A knife inserted near the center should come out mostly clean.

Let the frittata rest for 5 minutes before slicing. Garnish with fresh dill, parsley, or a pinch of red-pepper flakes if desired.