Place the first head of cauliflower upright (stem-side down) on a flat surface. Using a large knife and imagining that you are creating two large steaks from each head of cauliflower, trim a little off each end so that when you split the cauliflower in half, each half will lie flat. Cut the cauliflower in half. You should yield two "steaks," each weighing about 14 to 15 ounces. Repeat with the other cauliflower.
Bring 6 quarts of water to a rolling boil in a pot large enough to hold the cauliflower steaks. Add salt until the water tastes like mild seawater. (How will you know? Take a little water with a spoon and taste a drop of it.) Layer a baking sheet with a kitchen towel. Add the cauliflower steaks to the boiling water and cook until they are slightly tender when pierced with the tip of a knife, 6 to 8 minutes. Use a large slotted spoon or spatula to carefully transfer the steaks to the baking sheet to drain.
In a container large enough to fit the cauliflower snugly, whisk the coconut milk together with the coriander, red pepper flakes and a generous pinch of salt. Submerge the steaks in the coconut milk marinade and refrigerate for at least 2 hours and up to 24 hours.
For the vinaigrette:
Whisk together the lemon juice, mustard and vinegar in a medium bowl. Slowly whisk in the olive oil. Set aside.
For the gremolata:
Combine the parsley, lemon zest and garlic in a small bowl. Set aside.
Finish the cauliflower:
Preheat the oven to 375 degrees F.
Remove the cauliflower steaks from the coconut milk marinade and arrange them in a single layer on a baking sheet. Season with salt. Place in the oven and cook until tender, 10 to 15 minutes. Heat the broiler and place the cauliflower under the broiler for a few minutes, until the top chars. Transfer the steaks to a serving platter (or individual plates) and drizzle liberally with all of the vinaigrette. Top with all of the gremolata. Add a pinch more salt if desired. Serve immediately.