1. In a large sauté pan over medium heat, heat the olive oil and butter, and cook the onion, stirring frequently, until translucent, about 5 minutes
2. Add the mushrooms, season them with salt, and cook, stirring frequently, until they’ve released their juices and browned, about 10 minutes
3. Add another splash of olive oil, if needed, then stir in the spinach and basil and cook until wilted, about 2 minutes. Remove the vegetables from the heat and spoon them into a baking dish. Cool the mixture for 10 minutes or overnight.
4. When ready to cook, preheat the oven to 350 degrees F
5. If you’ve chilled the mushroom mixture overnight, warm it in the oven for 15 minutes before proceeding. Arrange the tomatoes over the mushroom mixture, then sprinkle the cheese over the veggies. Crack the eggs on top of the mixture, keeping the yolks intact. Season with salt and pepper.
6. Bake until the egg whites are set and the yolks are still runny, about 15-20 minutes. Note that the eggs will continue cooking a bit more once removed from the oven. Make sure you watch the eggs in the oven carefully…there’s a sweet spot when the whites are set but the yolks remain runny. If you cook a minute too long the yolks will harden.