Preheat the oven to 392F / 200CBrush both sides of the mushrooms with the oil then with the balsamic vinegar. Season with salt and pepper. Place the mushrooms stem side up on a small baking tray. Bake in the oven for 10 to 15 minutes.
Split the ciabattas in half, brush a little oil over each one then toast the inside of both pieces on a hot griddle pan. Spread the mayo over the bottom ciabatta.
When the mushrooms are roasted, carefully pat them dry if they are very wet.Place one mushroom on each ciabatta. Crumble over the feta then add the tomatoes and a couple of basil leaves.Place the remaining ciabatta on top on then serve immediately.
Nutritional information is approximate.
The size of your portobello mushrooms will determine how long they take to cook.
This is an easily scalable recipe, please see the serving size number in the recipe and slide it to the right to increase quantities.
If you want to make this recipe vegan, sub the mayo and pesto for vegan versions and sub the feta for a vegan alternative.
For a gluten-free version just use your favourite gluten-free bread and make sure the pesto is also gluten-free.