Halloumi Mushroom Burgers

• 1 tbsp extra virgin olive oil
• 2 portobello mushrooms, stems removed
• 1 brown onion, thinly sliced
• 100 g halloumi, thinly sliced
• 1 tomato, sliced
• 0.5 avocado
• 2 tbsp pesto
• 1 cup rocket leaves (arugula)
• 2 wholemeal rolls