Very lightly adapted from Martha Rose Shulman's The Simple Art of Vegetarian Cooking. It's a perfect, dead-simple brunch dish, not least because once you put it in the oven, you can tend to everything else that has to be done. I've included Shulman's melted onions recipe here, but feel free to substitute your own version of caramelized onions. (Or your own filling period. The template for the baked frittata is what's most important here.) And finally, if you don't have Greek yogurt, I've strained regular yogurt for just an hour or so before baking this. It's worked plenty well enough. —Nicholas Day
In a large skillet, warm the olive oil over medium heat and then add the onions and cook, stirring, until they soften, about five minutes or so. Then add 1/2 teaspoon sea salt, the thyme and the minced garlic. Turn the heat to low and cover. Cook, stirring frequently, for another 30 to 40 minutes, or until the onions are very soft and lightly colored but not browned.
Preheat the oven to 350 degrees F.
Add the olive oil to a large skillet, ideally cast iron, brushing the sides with the oil. Then place the pan in the hot oven for five minutes.
While the pan is heating up, whisk the eggs in a large bowl. Season with salt and pepper and whisk in the yogurt. Then stir in the melted onions (above), the walnuts and the parsley.
Remove the skillet from the oven and pour in the egg mixture. It should sizzle. Return the skillet to the oven and bake until puffed and lightly colored, about 30 minutes. Serve hot, warm, or at room temperature.