You can grill these delicious cauliflower "steaks" outside on the barbecue or indoors on a grill pan. The recipe also works well if you cut the cauliflower into florets and roast them in a single layer on a sheet pan.
To make the steaks: In a shallow bowl or resealable plastic bag, whisk together orange juice, oil, lemon juice, soy sauce or tamari, garlic, mustard, and red chili flakes. Season to taste with salt and pepper.
Carefully remove stem and leaves from cauliflower. Using a sharp knife, carefully cut entire head of cauliflower into “steaks” about 1⁄2-inch to 3⁄4-inch (1.3 to 2 cm) wide. (A large head should yield 5 or 6 steaks, with some florets left over. We call those “cutlets.”)
Add steaks, and any leftover cutlets, to marinade and refrigerate overnight.
Preheat grill or grill pan to medium-high heat. Add steaks in a single layer, reserving marinade for later use as glaze, if desired.
Allow steaks to grill about 5 minutes per side, or until grill marks are prominent and cauliflower releases easily from the grill. (If it sticks too much, it isn’t ready to flip.)
Flip and grill 5 minutes. Remove from grill.
To make the optional glaze: Place reserved marinade in a small pot and bring to boil. Reduce heat to simmer.
In a small bowl, mix together cornstarch and water to make slurry. Slowly pour slurry into simmering marinade and stir to thicken.
Remove from heat. Glaze will continue to thicken as it cools.