Step 1

Preheat oven to 350°. Toast walnuts on a rimmed baking sheet, tossing once, until golden brown, 7–10 minutes. Let cool, then coarsely chop. Set aside. Increase oven temperature to 425° for roasting cauliflower.

Step 2

Heat oil and capers in a small saucepan over medium, swirling often, until capers burst and are golden brown and crisp, about 5 minutes. Carefully pour oil and capers into a small heatproof bowl; let cool.

Step 3

Mix in walnuts, chile, parsley, currants, vinegar, and lemon zest; season with salt.

Step 4

Do Ahead: Salsa can be made 1 day ahead. Cover with plastic, pressing directly onto surface and chill. Bring to room temperature before using.

Step 5

Remove only the toughest outer leaves from cauliflower (leave on any tender inner leaves). Trim stem to create a flat base. Resting cauliflower on stem, cut in half from top to bottom, creating two lobes with stem attached. Trim outer rounded edge of each piece to create two 1½"-thick "steaks" (the stem should hold the florets together); set steaks aside. Very coarsely chop remaining florets.

Step 6

Cook florets in a large saucepan of boiling salted water until very tender, 6–8 minutes. Drain well, then process in a food processor with garlic, lemon juice, tahini, and 1 Tbsp. water until smooth; season with salt. Set aside.

Step 7

Heat oil in a large skillet over medium-high. Add butter and swirl until melted, then add cauliflower steaks, oregano, and lemon zest. Cook, gently lifting up cauliflower occasionally to let hot fat run underneath, until steaks are deep golden brown (if oregano or lemon start to burn, place on top of steaks). Turn steaks and season with salt. Transfer skillet to oven and roast until cauliflower stems are fork-tender, 10–15 minutes.

Step 8

To serve, spoon about ⅓ cup cauliflower purée onto plates and place steaks on top. Spoon salsa over and sprinkle with salt. Serve with lemon wedges for squeezing over.

Step 9

Do Ahead: Purée can be made 3 days ahead. Cover with plastic, pressing directly onto surface, and chill. Bring to room temperature before using.