Sweet potato, feta and red onion frittata recipe

Ingredients

• 300 g sweet potato, peeled and cut into 1cm (½in) cubes
• 1 tbsp butter
• 1 tbsp olive oil
• 2 red onions, very thinly sliced
• 1 garlic clove
• 1 tsp thyme leaves, plus small sprigs to serve
• 1 tbsp balsamic vinegar
• 1 tbsp flat-leaf parsley, chopped, plus extra to serve
• 6 large eggs, beaten and liberally seasoned
• 125 g feta