Massive mushroom burger

4 large portobello mushrooms caps, wiped clean

3 tbsp balsamic vinegar

1 tbsp olive oil

½ tsp salt

¼ tsp coarsely ground black pepper

100g plain low-fat Greek-style yoghurt

1 tbsp chopped fresh rosemary

1 lime, zested and juiced

Burger buns or rustic rolls, each cut in half

Handful lambs lettuce or rocket (depending on what your child prefers)

1 large tomato, cut into 8 slices

1 red onion, thinly sliced

3 roasted red pepper drained, cut into strips