Position to top oven rack in the middle position and preheat the oven to 400F. Line a baking sheet with parchment paper.
Remove the cauliflower's outer leaves and slice the stem. Rinse it under running water and dry it with a clean kitchen towel. Slice the cauliflower into ¾-1" thick slices.
Place the cauliflower steaks on the baking sheet. Lightly brush both sides of the cauliflower steaks with olive oil and season lightly with salt and pepper. You'll be adding the garlic sauce during baking.
Make the garlic mixture:
In a bowl, combine the olive oil, garlic cloves, salt, pepper, red pepper flakes (if using), and chopped parsley. Set aside.
Bake:
Bake the cauliflower steaks in the preheated oven for a total of 30 minutes. Set a 15-minute timer.
Halfway through, remove the baking sheet from the oven, flip the cauliflower steaks, and spoon about half of the garlic-herb mixture over the steaks, spreading it as a thin layer over the top. Continue baking the cauliflower steaks for an additional 15 minutes, until the middlemost part feels tender when poked with a fork.
Serve:
Plate the cauliflower steaks and spoon some of the remaining garlic-herb sauce over the tops.