Step 1: Prepare Your Kitchen
Preheat your oven to 400°F (200°C). Position an oven rack in the middle. Ensure your 10-inch skillet is clean and ready to go.

Step 2: Sauté the Aromatics and Vegetables
Heat the olive oil in your oven-safe skillet over medium heat. Once the oil shimmers, add the chopped yellow onion. Sauté for 4-5 minutes, stirring occasionally, until the onion softens and becomes translucent. Add the minced garlic and cook for another minute until fragrant, being careful not to let it burn.

Step 3: Cook the Heartier Vegetables
Add the sliced mushrooms and diced red bell pepper to the skillet. Continue to cook, stirring occasionally, for about 6-8 minutes. The goal is to cook off the excess moisture from the vegetables, especially the mushrooms, and allow them to get a little bit of color. This step is vital to prevent a watery frittata.

Step 4: Wilt the Spinach
Add the fresh spinach to the skillet in handfuls. It will look like a huge amount, but it will wilt down significantly. Stir continuously until all the spinach has wilted, which should take about 2-3 minutes. Spread the cooked vegetables evenly across the bottom of the skillet.

Step 5: Create the Egg Custard
While the vegetables are cooking, prepare the egg mixture. In a large mixing bowl, crack all 12 eggs. Add the almond milk, salt, black pepper, and garlic powder. Whisk vigorously for a full minute. You want to incorporate some air into the eggs, which helps create a lighter, fluffier texture. Once whisked, gently stir in half of the crumbled Feta cheese.

Step 6: Combine and Bake
Carefully pour the egg mixture over the vegetables in the skillet. Use a spatula to gently move the vegetables around so the egg mixture fills in all the gaps and reaches the bottom. Sprinkle the remaining Feta cheese evenly over the top.

Step 7: The Final Bake
Transfer the skillet directly into the preheated oven. Bake for 15-20 minutes. The frittata is done when the edges are set and golden brown, the center is just barely set (it shouldn’t jiggle much when you gently shake the pan), and it has puffed up slightly. A knife inserted into the center should come out clean.

Step 8: Rest and Serve
Carefully remove the skillet from the oven. Be extremely careful, as the handle will be very hot. Let the frittata rest in the skillet for at least 5-10 minutes. This allows it to finish cooking from the residual heat and makes it much easier to slice. The frittata will deflate slightly as it cools; this is normal. Garnish with fresh herbs, slice into wedges like a pie, and serve.