Step 1

Preheat oven to 400°. Stand Romanesco on its stem and cut it from top to bottom into 1"-thick slabs. Don’t worry about the outer edges that may crumble or smaller florets that break away; nudge those into a pile along with any nice-looking leaves. Cut loose bits into ½" pieces; season bits and Romanesco steaks with kosher salt.

Step 2

Heat a large cast-iron skillet over medium-high. Add 2 Tbsp. oil and slip Romanesco steaks into pan—puzzle them together so they don’t overlap; work in batches if needed. Cook, rotating skillet periodically for even browning, until dark brown underneath, about 5 minutes. Turn and drizzle a bit of oil over second sides and into empty areas of pan. Scatter chopped pieces of Romanesco into spaces between steaks and toss gently to coat with oil (add more if needed).

Step 3

Transfer pan to oven; roast Romanesco until a tester inserted into stems slips through without much effort, 15–18 minutes. Steaks should be firm enough to still hold together without being crunchy.

Step 4

Transfer steaks to a platter. Return pan with loose pieces of Romanesco to medium-high heat. Add butter, hazelnuts, and garlic, and cook, stirring, until everything is toasted and crisp around the edges, about 4 minutes. Add parsley and vinegar and stir up any browned bits. Spoon over steaks and season with sea salt. Serve with lemon wedges.