Preheat oven to 180C (160C fan-forced). Line a large baking sheet with non-stick baking paper.
Cut cauliflower in half, then slice into four 1.5 inch thick steaks. Remaining ends and florets can be frozen in a zip lock bag and added to soups, or turn them into cauliflower rice or mash before freezing. Trim the leafy ends off the stalks.
Whisk together the milk and apple cider vinegar. Place flour, arrowroot, salt and pepper in a bowl large enough to dip each cauliflower steak in. Mix well. Add the milk and apple cider vinegar mixture and whisk well to combine into a thick batter. Place breadcrumbs on a large plate and spread evenly.
Put each cauliflower steak into the batter. Coat evenly and shake off any excess batter. Place onto the breadcrumbs and press the steak gently to coat, flip and coat the other side. Set aside onto a cutting board. Repeat with the remaining cauliflower.
Heat about 2 tablespoons of oil in a large (non-stick, if you prefer) frying pan over medium high. Place 2 schnitzels in the pan and cook on each side for about 3 minutes, or until golden brown and crispy. You may need to add some extra oil to the pan when flipping the schnitzels. Once golden, transfer to the baking sheet and repeat with remaining schnitzels. Bake in the oven for 7-10 minutes.
Meanwhile prepare the sauce.
Peel and mince garlic. Heat the olive oil in a medium frying pan set over medium high heat. Add mushrooms, pepper and salt. Cook, stirring for about 5 minutes, stir in the garlic. Transfer mushrooms to a bowl and return pan to medium low heat. Add the vegan butter, add flour and whisk vigorously to make a smooth paste. Slowly whisk in the stock, the sauce should thicken. Add mushrooms and stir. Remove from heat.
Place salad leaves in a large serving bowl. In a small jug whisk together the olive oil, apple cider vinegar, rice syrup and salt. Dress the leaves with the dressing.
Remove cauliflower schnitzels from the oven and serve drizzled with the sauce. Serve salad leaves on the side.