Prepare a grill for medium-high heat and lightly oil grate. Drizzle cauliflower steaks, florets, and scallions with 4 Tbsp. oil; season with salt and pepper. Grill scallions, turning occasionally, until charred in spots, about 2 minutes. Grill cauliflower steaks until tender and charred in spots, 8–10 minutes per side. Grill any loose florets in a grill basket, tossing often, until cooked through, 5–7 minutes.