Using a sharp knife, slice each cauliflower head into 2cm/1inch steaks.
Brush the cauliflower with olive oil and season with salt.
Preheat an outdoor grill or grill pan over medium-high heat.
Add the cauliflower to the grill grates and cook for 8-10 minutes per side until the cauliflower is cooked through and the stems are tender with grill marks on both sides.
While the cauliflower grills, stir the chopped parsley, red chillies, crushed garlic, red wine vinegar, olive oil, dried oregano, salt and pepper together in a small bowl.
Taste and adjust seasoning if necessary by adding a few extra teaspoons of vinegar, salt or herbs.
The sauce can be made up to a day in advance and kept in an airtight container in the fridge.
Once cooked, remove the cauliflower from the grill and place on a serving plate.
Drizzle the chimichurri over the cauliflower and serve.