Position an oven rack approximately 4 inches from the broiler and heat oven to the high broil setting.
Rinse and trim any green leaves from the head of cauliflower, leaving as much of the center stalk as possible. Lay the head, stem up, on a cutting board and cut in half from top to bottom. Lay each half on its side and cut in half again. Slice each quarter into pie-style wedges, so that a piece of core holds each wedge together and the outer edge thickness is not more than 3/4-inch thick. Lay the cauliflower onto a wire rack set inside a half-sheet pan and cover with aluminum foil, leaving one end open.
Crush together the cumin, coriander, mustard, orange peel, turmeric, and fenugreek in a small ramekin using the end of a rolling pin. Place the oil in a large metal measuring cup or 1-quart saucepan and set over high heat until it shimmers. Immediately remove from the heat, add to the spice mixture, and steep for 5 minutes.
Place the half-sheet pan into the oven and carefully pour in the water, seal the foil opening, and broil for 6 minutes. Remove the foil and brush the cauliflower with half of the seasoned oil. Sprinkle with 3/4 teaspoon salt. Return to the oven and broil until browned, 5 to 10 minutes. Turn the cauliflower over and brush with the remaining oil and sprinkle with the remaining 3/4 teaspoon salt. Return to the oven and broil until browned, another 5 to 10 minutes. Serve immediately.