Preheat the Oven: Preheat your oven to 350°F (175°C).

If not already cooked, cook the bacon in a skillet over medium heat until crispy. Remove, let cool, and crumble.

In an oven-safe skillet, heat the olive or coconut oil over medium heat. Add the chopped onion and sliced mushrooms, and sauté until the onions are translucent and the mushrooms are tender, about 5-7 minutes.

Add the baby spinach leaves to the skillet and cook until wilted, about 2 minutes.

In a large bowl, whisk the eggs and season with freshly ground black pepper.

Pour the eggs over the sautéed vegetables in the skillet. Sprinkle the crumbled bacon evenly over the top.

Let the frittata cook on the stovetop for 2-3 minutes until the edges begin to set. Then, transfer the skillet to the preheated oven.

Bake: Bake for 15-20 minutes, or until the frittata is fully set and lightly golden on top.

Remove from the oven, let cool slightly, and slice into wedges to serve.