Sautéed Cauliflower Frittata with Thyme

unsalted butter

extra-virgin olive oil

medium head of cauliflower cut into 1-inch florets (2 cups)

Sea salt

large onion, halved and thinly sliced

small garlic cloves, thinly sliced

large eggs, at room temperature, lightly beaten

dry bread crumbs from 1 piece of toast

thyme leaves

Freshly grated Parmesan cheese

Truffle salt or fleur de sel and aged balsamic vinegar, for serving