1. 1. Saute the Aromatics and Vegetables: Heat the olive oil in a medium, oven-safe skillet (preferably cast iron or another non-stick, oven-safe pan) over medium heat. Add the chopped onion and cook until softened, about 3-4 minutes. Stir in the minced garlic and diced red bell pepper, cooking for another 3-5 minutes until the bell pepper starts to soften.

2. 2. Add the Greens and Mushrooms: Incorporate the chopped spinach and sliced mushrooms into the skillet. Cook, stirring occasionally, until the spinach has wilted and the mushrooms have released their moisture, which usually takes about 3-5 minutes. Remove the skillet from the heat.

3. 3. Prepare the Egg Mixture: In a medium bowl, whisk together the eggs, milk, salt, and black pepper until well combined and slightly frothy.

4. 4. Combine and Add Feta: Evenly distribute the cooked vegetables in the bottom of the skillet. Sprinkle the crumbled feta cheese over the vegetables. Pour the whisked egg mixture evenly over the vegetables and feta in the skillet.

5. 5. Initial Stovetop Cook: Place the skillet back on the stovetop over medium-low heat. Cook for 5-7 minutes, without stirring, until the edges of the egg mixture begin to set and pull away from the sides of the pan. The center will still be quite liquid.

6. 6. Finish in the Oven: Carefully transfer the skillet to a preheated oven at 350 F (175 C). Bake for 12-15 minutes, or until the eggs are fully set in the center and lightly golden on top. A knife inserted into the center should come out clean.

7. 7. Rest and Garnish: Remove the skillet from the oven and let it rest for 5 minutes before slicing. Garnish with fresh chopped parsley, if desired, before serving.