AVOCADO, CHEESE AND MUSHROOM QUESADILLAS

2½ tbsp olive oil, plus extra for frying • 250g mushrooms (such as button, chestnut and/or portabella), thickly sliced • 2 garlic cloves, crushed • 3 thyme sprigs, leaves picked • 2 large avocados, halved • ½ lemon, zest and • 1 tbsp juice • 4 large flour tortillas • 80g grated mozzarella • 100g extra mature cheddar, grated • soured cream, to serve