Preheat oven to 400ºF and line a rimmed baking sheet with parchment paper.
Using a sharp chef's knife, trim the leaves from the bottom of the cauliflower and cut off the very bottom of the stem. Cut the cauliflower in half down the middle, through the stem. Take one half of the cauliflower and cut it into ¾ to 1-inch slices—you should end up with 2-3 steaks, with some florets falling from the sides. Repeat with the other half of the cauliflower.
Whisk together the egg and milk in a wide, shallow bowl with enough room to dip the cauliflower steaks. Pour the flour onto a large plate and combine the panko, parmesan, mustard powder, salt and pepper on a second large plate.
Dredge the cauliflower steaks in the flour, dip them in the egg mixture, and then press them into the panko mixture. Use your hands to make sure the cauliflower is fully coated on all sides with each. Place the steaks on the prepared baking sheet and spray liberally with oil.
Bake the steaks for 15 minutes, then flip them over and spray them with more oil. Bake for another 15-20 minutes, or until golden brown with tender florets. Sprinkle with chopped herbs and a squeeze of lemon juice before serving.
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