1. Preheat the oven to gas 7, 220°C, fan 200°C. Pulse the torn bread in a food processor to fine breadcrumbs, tip onto a plate and stir in most of the oregano. Put the egg and flour in separate bowls; season the flour.
2. Working in batches, dust the aubergine rounds in the flour on both sides, then brush with egg until completely coated. Dip into the breadcrumbs, pressing them into the aubergine to stick. Transfer to a lined baking sheet. Once all the aubergine rounds are breadcrumbed, drizzle over 2 tbsp oil, then bake for 20-25 mins, turning halfway, until golden and crisp.
3. Meanwhile, mix the cucumber, yogurt and garlic with some black pepper. Split the sub rolls in half, keeping one side attached. Spread 1 heaped tbsp of houmous on each base and top with the spinach. Stuff the crispy aubergine slices into the rolls and top with the red onion, a dollop of the tzatziki and a crumbling of cheese. Toss the sliced tomatoes with the remaining oregano, any tomato juices from the chopping board and 2 tsp olive oil. Serve alongside the sub rolls.
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