Low-carb turkey burgers with kumara 'buns'

Ingredients

• 1 large kumara, unpeeled (see tip)
• 1/3 cup quinoa flakes
• 1/4 cup milk
• 1 courgette, coarsely grated
• 1 small carrot, coarsely grated
• 1 small red onion, coarsely grated, plus 1 red onion extra, thinly sliced
• 400 grams turkey or chicken mince
• 1 tablespoon parsley, finely chopped
• 2 tablespoons olive oil
• 1 large tomato, thinly sliced
• 100 grams trimmed watercress
• 1 tablespoon toasted sesame seeds
• Green tahini
• 1/4 cup tahini
• 2 tablespoons parsley leaves
• 2 tablespoons lemon juice
• 1 tablespoon olive oil
• 1 small garlic clove, crushed