Preheat oven to 350°. Line rimmed baking pan with parchment paper. Place cauliflower, stem side down, on cutting board; cut four 3/4-inch-thick steaks from center (reserve remaining cauliflower for another use).
Place steaks on prepared pan; coat both sides with oil and sprinkle with salt and pepper. Bake 20 minutes, turning once halfway through cooking.
In large skillet, in 2 batches, cook 4 thyme sprigs, 4 garlic cloves, 2 rosemary sprigs in 2 tablespoons butter over medium-high heat 2 minutes. Add 2 cauliflower steaks; sear 2 minutes on each side, basting occasionally with butter mixture. Repeat to sear remaining 2 cauliflower steaks. Serve steaks drizzled with butter mixture.