Whole baked cauliflower with tomato & olive sauce

INGREDIENTS

• 1 red onion, peeled and sliced
• 5 cloves garlic, peeled and chopped
• 1 large head cauliflower, outer green leaves discarded, stalk chopped
• olive oil
• 1 handful black olives, stoned
• 4 good-quality salted anchovy fillets in oil, drained and sliced
• 1 handful fresh flat-leaf parsley, leaves roughly chopped, stalks finely chopped
• 2 x 400 g good-quality tinned plum tomatoes
• red wine vinegar