Set a steamer basket inside a large, deep pot, and add water to a depth of ½ to 1 inch – just below the bottom of the steamer.
Rinse and trim the cauliflower. Place them in the steamer, cover, and steam for 15-20 minutes.
By this point, they should be tender and should have changed in color from a raw, opaque white to a slightly more translucent, yellowy off-white.
Meanwhile, combine the oil, lemon juice, salt, pepper, and nutmeg in a small bowl, whisking to mix well.
When the cauliflower is ready, gently transfer them from the steamer to a medium baking dish or small roasting pan, something just large enough to hold the two heads side by side.
Using a pastry brush, paint the cauliflower with the oil mixture. Cover the pan with aluminum foil, and place it in the oven. Roast the cauliflower for 30 minutes, basting every ten minutes. [You should have just enough of the oil mixture for three light bastings in total, including the first one.]
Remove the foil, and continue to bake for another 10-20 minutes, until the cauliflower is pale golden and a knife can be easily inserted into its core.