Preheat oven at 200°C. In a large ovenproof pan, on medium heat, melt the butter and add the onions and sauté until they have lightly caramelised. Add the chopped bell peppers and mushrooms, and sauté until they have softened. Add the baby spinach, and sauté until they have wilted. Season with salt & pepper.
In a large bowl, beat the eggs and season with salt and pepper. Pour the mixture over the sautéed vegetables and evenly sprinkle the feta cheese on top. Cover and place the pan in the oven and cook for 8–10 minutes until the eggs are done.