Sauté Aromatics: Warm 3 tablespoons of the extra virgin olive oil in a large, oven-safe skillet (approximately 10-12 inches / 25-30 cm) over medium heat. Add the thinly sliced onions and cook, stirring occasionally, for 10-12 minutes, until they are well-softened and translucent. Add the minced garlic and red pepper flakes (if using) during the last minute of cooking, stirring until fragrant.

Wilt Greens: Add the fresh spinach to the skillet with the onions (you may need to add it in batches). Stir gently for 4-6 minutes, until all the spinach has wilted down. Season the vegetable mixture generously with the measured salt and black pepper.

Prepare Egg Mixture: While the vegetables cook, crack the 6 large eggs into a separate large mixing bowl. Beat them lightly with a whisk or fork. Gently fold in the 3/4 cup ricotta cheese and 6 tablespoons grated Parmesan cheese until just combined (avoid overmixing).

Combine and Cook: Carefully transfer the cooked onion and spinach mixture from the skillet into the bowl with the egg and cheese mixture. Stir everything together until evenly distributed.

Initial Cook: Wipe out the skillet if necessary, then add the remaining 1 1/2 tablespoons of extra virgin olive oil, ensuring the bottom is coated to prevent sticking. Return the skillet to low heat. Pour the combined egg and vegetable mixture back into the prepared skillet, spreading it evenly.

Set the Bake: Cover the skillet with a tight-fitting lid. Cook on low heat for 12-15 minutes. The edges should appear set, and the top should be mostly set but still slightly jiggly in the very center.

Flip and Finish: Carefully loosen the edges of the egg bake with a spatula. Place the lid (or a large plate) firmly over the skillet and confidently invert the bake onto the lid/plate. Gently slide the bake back into the skillet, uncooked side down. Cook, uncovered, for another 5-7 minutes, allowing the second side to become firm and golden brown.

Serve: Slide the finished Rustic Spinach Ricotta Skillet Bake onto a cutting board or serving plate. Let it rest for a couple of minutes before slicing into wedges. Serve warm or at room temperature.