Step 1Bring to a boil a pot of salted water large enough to fit the cauliflower. While the water is heating, trim about an inch of the core from the cauliflower and remove any large leaves, leaving the smaller ones intact. Through the core, cut the cauliflower into 6 wedges.
Step 2Add the cauliflower to the pot and cook just until slightly tender, 2 to 4 minutes. Remove the cauliflower and place it in a bowl of ice water to stop the cooking. Remove and pat dry. Season each piece with a pinch of salt and pepper, or to taste.
Step 3Heat a heavy-bottomed sauté pan (preferably cast iron) over medium-high heat. Add the oil to coat the bottom of the pan. Add the cauliflower pieces and sear them on each of the two cut sides until nicely browned, 6 to 8 minutes. Transfer the cauliflower to a serving platter.
Step 4Spoon some of the warm bagna cauda over the cauliflower, placing the rest in a bowl to serve on the side. Garnish the cauliflower with chopped parsley. Using a lemon zester, zest strands of lemon over the cauliflower.