How to Make It
Preheat the oven to 400°. In a 10-inch, nonstick, oven-proof skillet, melt 1 tablespoon of the butter in the olive oil. Add the cauliflower florets, season with sea salt and cook over high heat, without stirring, until golden brown on the bottom, about 2 minutes. Toss the cauliflower florets in the pan and cook until golden brown all over and just-tender, about 3 minutes longer. Add the sliced onion and 1 tablespoon of the butter and cook, stirring occasionally, until the onion is softened, about 3 minutes. Reduce the heat to moderate, add the garlic and cook until it is softened, about 1 minute longer. Add the remaining 1 tablespoon of butter and swirl the pan to melt it.
Meanwhile, in a large bowl, whisk the eggs with 3/4 teaspoon of sea salt. Pour the eggs over the cauliflower and cook, without stirring, until the bottom of the frittata is just set, about 2 minutes. Lift the edges of the frittata and tilt the pan to allow the uncooked egg mixture to seep underneath. Continue cooking the frittata until the bottom is golden and the top is just slightly runny, about 3 minutes.
Sprinkle the bread crumbs and thyme over the frittata and bake until it is just set, about 3 minutes longer. Sprinkle the frittata with freshly grated Parmesan cheese and slide it onto a work surface or platter. Cut the frittata into wedges and serve warm or at room temperature with truffle salt and balsamic vinegar.