1. Preheat the oven to 375º F.
2. Combine the onion, vinegar, and ½ teaspoon salt in a bowl and mix well; set aside.
3. In a large mixing bowl, toss the cauliflower with the turmeric, 2 teaspoons of oil, ½ teaspoon salt, and black pepper.
4. Preheat a 12-inch ovenproof skillet over medium heat. Add 1 teaspoon of oil and the cauliflower, tossing once or twice, and cook until the edges are golden brown, 8-10 minutes. Transfer to a small bowl or plate and set aside.
5. Whisk the eggs, mustard, ½ teaspoon salt, and black pepper in the mixing bowl used for the cauliflower.
6. Heat the remaining 2 tablespoons of oil and the butter in the skillet over medium-high heat until the butter melts. Add the onion-vinegar mixture and cook, stirring occasionally, until most of the liquid has evaporated and the onion is beginning to turn dark golden brown, 5-8 minutes. Adjust the temperature accordingly to avoid splattering of hot oil. Once the onions are done, remove the skillet from the heat, stir in the cooked cauliflower, and stir to ensure the cauliflower and onion is evenly distributed; slowly pour in the egg mixture.
7. Spoon the cheese on top in dollops from the outside inward, circling around and covering the entire surface.
8. Bake until the eggs are set in the center, about 15 minutes.
9. Let rest for 5 minutes and then slice into wedges. Top with the optional cilantro or parsley and serve.

Source: Karen Falbo