Preheat the oven. Set to 350°F (175°C).
Lightly grease a standard 12-cup muffin tin with nonstick spray, or use silicone liners to prevent sticking.
Prep the spinach. Heat olive oil in a skillet over medium. Add garlic, sauté 30 seconds, then add spinach. Cook 1–2 minutes until wilted.
Remove from heat and let cool slightly. If using frozen spinach, skip the skillet and just squeeze out excess water.
Whisk the eggs. In a large bowl, whisk eggs with milk, salt, pepper, and oregano or dill if using. Whisk until mixture looks uniform and lightly frothy for fluffier muffins.
Add the mix-ins. Stir in the chopped spinach and crumbled feta.
Reserve a small pinch of feta to sprinkle on top if you like a golden finish.
Fill the muffin cups. Divide the mixture evenly among the 12 cups, filling each about 3/4 full. The mixture will puff in the oven and settle as it cools.
Bake. Place the tin on the middle rack and bake for 16–20 minutes. The muffins are done when the centers are set and a toothpick comes out with no wet egg.
Cool briefly. Let the muffins rest in the pan for 5 minutes, then run a thin knife around the edges to release.
Transfer to a rack to cool, or serve warm.
Serve. Enjoy as is, or add a drizzle of hot sauce, a spoonful of salsa, or a side of sliced tomatoes and avocado.